Okay, to touch further upon what Flam said, pop music =
popular music.
If it is available for purchase from a major label as well as many independent labels and sells, it is popular music. Pop is a broad umbrella that covers the vast majority of the genres of music most listened to by members of this forum.
From a biological perspective, we could look at art as the Domain and music as one of the Kingdoms. Popular music might fall into an Order with genres being the Families (and subgenres being the Subfamilies) or you can choose to go further down the line and place popular music as a Genus and the genres as Species. Either way, "pop" music encompasses almost any genre you can think of regardless of the scope of said popularity.
The Beatles were certainly pop, but so were Zeppelin, with rock, hard rock, blues folk, and further rockabilly, Amglo-Celtic folk being among the variations they played/dabbled/stole

thu

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I find the biggest difference being the process. Peter Gabriel is "pop" artist that used to do progressive rock and then got more into world music...which was really always pop, but he, Paul Simon, Malcolm McLaren, and others helped expose "world music" to a wider audience (because white people love a vanilla coating to their music). This made it more popular.
Classical music and it's many phases or eras were also the popular music of their day despite popularity being more insular given the limited access to much of the higher art by the masses. Folk music is arguably the oldest popular/pop music. It was/is music by and for the common people and is wonderfully diverse as we all know.
Modern popular music is as broad in scope as it always has been, but the focus of promoting and marketing has made the music secondary to the "artist(s)". The whole 360 deal as a concept makes it about marketing as much of the artist as possible to maximize profits from any and all ability they have to generate revenue. Unfortunately, I think the music projects that. Most of the stuff that has been hits for the better part of the last two decades seems passionless. So much of it is so homogenized and devoid of uniqueness. And it's not about selling image as there are tons of amazingly talented and beautiful people making excellent and even awe-inspiring music that aren't getting a fraction of the attention and exposure of people that clearly less musically able or even committed.
The modern pop artist is a part of machine that refuses to die despite endlessly shooting itself in the foot. If you want to be famous and actually make money you have to give up any notion of control and personal creativity and let the machine process you for the masses. It's disgusting. The focus is to minimize and eventually eliminate failure, create an always successful formula, but as we've all read about it doesn't always work. This idea also misses out on the growth that can come from failure or that commercial success/failure has nothing to do with artistic success/failure...they can be mutually exclusive. We've all seen amzing movies that were "bombs" and tons of shite movies that were "blockbusters".
Anyway, I'm done ranting. Good day to you all.