Answer:
Pop music is an abbreviation for popular music. In other words, the main difference between pop music and alternative or underground music is the kind of following it has: pop music is usually known by a lot of people and regularly appears on TV. The artists are usually known as celebrities, and not just musicians.
On a musical level, the difference is usually that pop music is simpler. For example, a lot of pop songs follow a simple four-chord structure, and usually aren't very original on a lyrical level either. Of course, there are exceptions where a song that's more complex does hit the charts and become popular, but overall, pop songs are simple, sing-along songs.