Nowadays there are many genres and subgenres of music: so many it sometimes too hard to understand what each genre even about or how say trash metal differs from first wave of death metal or second wave of the same.
That was not always like that, there were times where there were just two genres and even just one. However eventually they split up into so many different ones.
A brief dive into history of contemporary music would allow you to understand why it happened as well as better understand genres.
Pop
At the beginning there was just pop. Pop is short for popular and that was its intention and agenda: become as popular as possible. Before pop there was jazz, blues, bebop and many more; before these there were classics like Mozart and Bach. However, you would not call them contemporary. They have their own history that needs their own article.
To that end pop had catchy tunes with simple lyrics, that were often about love. Songs were typically performed by a good-looking singer who sometimes danced but almost never played any instruments. Physical appeal of singer's looks was often more important than quality of songs or their singing ability. That was further enhanced with selling posters and various merchandise with the likeness of the artist. Producers clearly counted on singer's appearance being the selling point of the whole outfit. The genre revolves around this single singer or sometimes a group of singers who are called stars, idols or celebrities.
To support the singer there was often huge crew of various assistants. Musicians, dancers, make-up artists and so on. Professional composers wrote music and lyrics, professional arrangers arranged it. Professional dancers danced and professional musicians played. All of that was managed by producers. The singer was only the face of the huge machine and the industry behind it.
With some borrowings here and there, mixed with token efforts to change, that are superficial in nature, this genre continues to exist to this day. It is the genre that "keeps changing in order to stay the same".
Its popular mostly with women. Female psychology is wired to like popular things, and it is what keeps this industry running. That applies equally to both female and male pop artists. Male pop singers are popular because their physical appearance appeal to female listeners. Women wrongly assume that female pop singers should be equally popular with male audience but that it not the case. Female singers are also becoming famous through their appeal to female listeners. Women watch and listen to a female idol; assume she is popular with males and dream of being as popular and loved by guys as she is. Reality is that typical pop artist is only popular with other females most males are indifferent to most pop singers.
Rock
Rock was the second genre to emerge. Meaning of its name is ambiguous. Maybe it's from the verb rocking, because it shakes the industry and establishment. May be its from phrase rock solid, because it's not as fake as pop. Rock is not popping as bubblegum bubble, its fully solid. May be its short from Rock'n'roll dance and related music.
Rock emerged in protest to pop and aimed to be everything pop was not. Rockers blamed pop for being fake and wanted to be true and real. To that end they would write their own songs and lyrics, play them on stage and do everything else themselves. Unlike pop they will not have anyone behind the scenes. To credit everyone who contributed to the effort and not just the singer, they would organize themselves in bands. Often such bands would have catch names. To show they are not faking it and actually able to play instruments and sing, they would perform life on stage. Often there would be one dedicated singer, but some bands, like Beatles would take turns singing.
Songs and lyrics were different too. Not satisfied with sanitized and docile lyrics only about love, rock would expand topics, war, politics, social issues and many more. Often, they would have deliberately controversial lyrics to protest something they dislike about society. Sometimes they would take deliberate jabs at pop, mocking the genre for its various traits, rockers have issues with.
Melodies were similarly different, rock had less musicians to work with, but they were often willing to experiment with instruments and produce more radical and never before heard sound. Often that sound would be much heavier and angrier in its character compared to pop. Other times desire to showcase personal provenness as musician and ability to play would make certain musicians to compose and play complicated solos. Jimmy Hendrix particularly famous for it.
All of that made Rock both much more diverse and sophisticated than pop.
It also brought love of fans who disliked pop for one or the other reasons. Gradually rock had about as much fans as pop and most of these were new people who used to shun pop. Who had more fans at any given time is too hard to evaluate now.
However, many rockers are remembered much more roundly and by many more people compare to their pop equivalents. Pop was always in the now and artists who stopped releasing albums or performing would often be forgotten soon enough. In contrast rock had legends that endure long after they either disbanded or even literary died. Beatles, Kurt Cobain of Nirvana, Viktor Tsoi of Kino are ever remembered by their diehard fans, something pop artists cannot brag about. Unlike pop, rock could create this lasting voice of generation effect, not just fashionable this year passing fancy.
Because of rock, term rockstar rather than popstar is now associated with musical success, fame and things that go with it.
However, pop continued to be popular with its core demographics. It was not that its formula did not work, it was that it did not work on everyone.
Synthesis
Hoverer while rock build itself around the rejecting everything pop was, that feeling was not mutual. Far from it, pop actually say rock as a form of pop, that does the same thing as them, even if in a somewhat different way. That sentiment would make more purist rockers churn and spit. However, being popular was good enough credo in the eyes of pop industry and rock certainly was popular.
Because of that pop decided to imitate rock and learn its secrets in order to be even more popular than before. Thus, pop rock emerged where pop artists would adopt look of the rock bands and would sing holding guitars and such. That did not manage to win any rock fans over to pop but was popular enough with the pop crowds for several bands to do it.
What had much more enduring and lasting success however been rock musicians going pop. Sure, it was unacceptable from rock purists' point of view, but pop had something to entice popular rockers and that something was money. Sure, purists would turn back on you, calling you a sellout to big corporations' dirty money. However, so what if your old friends, with whom you started your career now curse you when you are millionaire and living a blast of a live.
There were even natural tendencies towards this development. No matter how much rock wanted to share fame and credit equally among all band members, public would still remember the lead singer much better than the guy who sit behind the drums. They will see him as leader and the rest as followers. That would allow "leader" to ditch his bandmates for hired musicians and a career under their own name.
Even Beatles, where all four members composed songs and lyrics and took turns singing were not immune it that tendency. Public grew to see John Lennon as leader, Paul McCarthy did not want to stay in Lennon's shadow, so he quit Beatles to start solo career. Much to McCarthy charging, Lennon remained the most popular of four in his solo career as well.
That happened many times over, Black Sabbath grew both Ozzy Osbourne and Dio, who after being vocalists there for some time, quit and continued as solo musicians. There are many more solo musicians, who started in a band that few remember nowadays and then quit to become solo artists. Sting is a good example.
Such "gone solo" musicians would be best equipped to be successful in both worlds. Rock origin would allow them to keep their rock credo together with many of the original fans. They will also have unique inside knowledge of rock world and what it takes to be successful with the rock fans.
Because of difference in attitudes towards each other, pop world would welcome such defector with open arms, and they will have a head start towards stardom.
To keep their original rock fans from dissipating, many would continue to perform with their new hired band on stage. They will also make their new songs sound as close to those of their original band as possible.
To keep pop fans engaged, there will be more show on stage interviews posters and other merchandise.
Do this right and your career would be as never-ending as that of Ozzy Osbourne and you will be super-rich embodiment of a rockstar. That is far more that what pop rock could possibly achieve, no one could possibly remember a name of a successful pop rock musician.
Diffusion
While rock fans are united in their mutual dislike for pop, when it comes to what they like about rock they are not united at all.
Many rebel movements, that succeed in overthrowing the regime, end up splintering into several smaller groups, that each fight for their distinct vision for the country. Sometimes these are political fights like in the US. between states and federal government and them between federalist party and democratic-republican one. Other times it is a continued armed struggle, like in Libya between Islamists and secularists.
Same with rock. Once unified movement eventually begun branching out into several different movements, who themselves later developed many subgenres of their own.
The most prominent offspring of rock is of course metal. Metal was a logical continuation of enhancing the sound. Metal takes that hard rock sound to its complete extreme. Even early rock often had heavier sound than pop with guitar riffs, distortion and many other sound effects that made sound harder and angrier. That branch first became hard rock, then metal and finally branched out into innumerable subgenres of said metal.
Each subgenre of mental differs in lyrics, appearance, ideology. Some sing about death, others about violence, some about occult stuff or devil, others about legendary warriors or even fantasy worlds They have differences in sound as well, but all of them sound much heavier and angrier than pretty much every other genre. Metal also developed a specific form of singing, called growl, that half of metal bands use. Other instead scream or yell and some sing in more normal voice.
Metal fans are typically people angry antisocial people. Some want to at least listen to songs about destroying society if they cannot actually destroy it. Other instead indulge themselves into escapism about legendary warriors, distant lands and fantasy worlds. Leather jackers, aggressive colors (particularly black) as well as aggressive fonts are typical among both fans and bands.
Metal fans are less concerned with opposing pop compared to expressing their anger with society, so they are not opposed to commercialization. Certain metal bands, that clearly begun as commercial projects, are as popular as more underground ones. Normally, metal does not deliberately aim to be an affront to social norms but often ends up being one due to being generally as extreme as possible.
Other much smaller branch decided to double down on ethics of being authentic and not selling out. They came to not only oppose pop as genre, but also popularity in general. They double down on lyrics and sing about issues they find meaningful for them. In their lyrics they often criticize pop, society and entertainment industry in general. Skyclad is an example of such band.
Unlike metal, who takes sound to the extreme, such bands have more toned-down sound and emphasize lyrics instead.
As much authentic rock would rather "be bitter than a flavor of the week", pop and hipsters came enjoy imitating them and their anti-popularity stance and created various indie and hipster bands to make dubious claims to authenticity.
Hister bands deliberately try to go against trends in hope that would make them popular instead. While fans of authentic rock would scoff and these hipster efforts, these efforts actually worked for some of them, and they gather support from likeminded fans who listen to such bands in order to look edgy and cool to their peers. For hipster it does not matter if lyrics matter, or band just act this way.
Then there are, people who like to be rebels without a cause. They gravitate towards various forms of punk rock. Unlike metal that takes sound to extreme, punk rock instead takes debauchery and chaos to the extreme. To the extend it can be contrasted with both metal and authentic rock, because if metal cares for cause more than for rebellion then punk does not care for a cause at all and just wants to rebel for rebelling sake.
To that end punk is deliberately as offensive to public tastes as they manage to get. Appearance wise punk tries to stand out as much as possible. Unlike stubborn defiance of metal, punk is all about screaming and shocking. Piercing, colored hair, torn clothes and such are very common among these group.
I punk are rebels without cause, then metal is cause without rebels.
There are other offshoots of rock, such as psychedelic rock of Pink Floyd, that is liked some people due to their possibly eerie sound. Unlike metal they try to make their sound as strange as possible.
There are some that combines certain elements of these and other genres such as folk or even soprano singing to make unique sound and create a new genre in their own right.
In addition to these and other offshoots of rock, there are many new genres that emerged completely on their own without particular connection to either pop or rock, techno for example.
Finally, there is rap, for little girls who are into black people. In that regard its closer to pop than to rock, but it's not completely so.
Conclusion
Conflict between pop and rock ultimately produced diverse and multicolored contemporary music scene, where everyone can find something to their liking.
The common pattern between these developments can be observed in other events of the era. A new trend first unites supporters around itself, then produces opposition. Opposition typically wins but then splinters into several distinct groups. Eventually field diffuses into variety of different groups with many diverse characteristics. Many of these new groups combine traits of the original and the opposition in different proportions.
For example, cold war begun because communism was created. Communists were one and united for a common cause and common ideology. In contrast their opponents were very different from one another and only united in their common dislike for communism. When communism was defeated, many went on to develop in their own, sometimes very different way.
More recently war in Libya showed similar trend.
Something less geopolitical is feminism. Feminists are united for the common cause, but opponents of feminism do not have common vision of what they want from relationship, only common dislike for what feminism is.
The principle of "thesis, antithesis and synthesis" works in all of these cases.
