Often I have students that have a big question:
What's the difference between musical styles?
In other words, what makes a "Rock" song a "Rock" song and what makes a "Country" song a "Country" song, and what makes "Reggae" music "Reggae" music or "Jazz" music "Jazz" really ?
That is a great question, and the answer is always changing over time. Let's explain the basics though!
#1 Traditionally music was identifiable by region and learned locally from family or the small town you were in.
This meant that way back before cheap transportation (cars, planes etc) musicians didn't relocate as often as they do now. Also music was quite often something that was a primary form of entertainment (before Radio if you wanted to hear a song, well you either had to play it yourself or know someone who did). Much like evolution, when things are isolated, they develop in unique ways.
This created different unique styles of music that often varied, sometimes as specific from one town to another. This is why when some people talk about a specific kind of music genre, like the Blues, they may specify the sort of blues "Chicago Blues" or "Delta Blues" for example.
The differences however become much more drastic based upon the places someone's traditional culture may be from, as again, way back when before Radio, or TV there was really no other way to learn music than from one's tight knit family or other people in the town you were in. Much of the music landscape today can be traced easily back to the early 1900's and even much further. In America, the landscape culturally and geographically was very very vast. It was easy for small pockets of certain kinds of music to develop on their own, isolated from one another .... until Radio became a major form of entertainment and listening to music.
#2 Music Genres were invented partially as a need to identify the sort of recordings or radio programs that begun to exist.
When Radio became something that people could listen to in their homes, of course music was very very popular. And different Radio Stations would often broadcast the music from their specific areas (because that was all there was in that area). But something interesting happened: People heard other music for the first time they had never heard before! And they knew it was different than one sound they had been hearing. There needed to be names that were more specific for broader genres.
This is how the music genres of Rock, Rhythm and Blues, Blues, Rockabilly, Hillbilly, Western, Country, Country Swing, Do Wop, Pops, Jazz and so many others came to be separated. By subject matter and tonality.
#3 Basically though, it often comes down to Rhythm, tempo and tonality.
A Pop / Songwriter song can be played as a Punk Rock song (like Paul Simon's Me and Julio being Covered by Me First and the Gimme Gimmes), and by that I mean a song can be played in a variety of styles, but in reality the difference between the styles in this modern era is the tonality of the song (the kinds of instruments used) and the underlying Rhythm of the song. The Rhythm tends to be most important, as originally, different sorts of music identified specifically with different sorts of Rhythm or Rhythmic Accents. This is the difference between many styles of Latin Music ... interestingly each one has a specific dance the coincides with it that highlights the special accents in the rhythm. Yes Rhythm is really that important.
#4 The Exception:
In this day and age, being specific to styles can be very confusing: often there are pop singers being called country, that arent' from the "country" and don't sing anything like a traditional country singer. This makes things rather confusing ... but as I originally said, the definitions of music are changing all the time. Since we are in a world where we now can discover thousands of new sounds in a matter of a few hours, a process that once literally was impossible until the last decade, we now have music that can have a Hip Hop Beat, with Jazz Vocals, with Country Fiddles, and Folk music percussion, and it is being sung in Japanese.
At this point, all the boundaries to different styles are open.