In the broadest sense, "pop" is short for popular. Popular music historically is music made by and for the lower classes, in contrast to "classical" music which required symphony orchestras and was intended solely for the well-to-do. Basically, popular music covers rock, rap, jazz, blues, and any and all derivatives thereof.
Since such a broad term is completely useless, "pop" has been reassigned to mean that radio-ready overproduced vocal stuff everyone seems to hate but still manages to sell ten trillion copies in its first fifteen minutes. Although there's also power-pop, covered by Matthew Sweet and Teenage Fanclub and such, and indie pop, such as the Shins and the New Pornographers. So it's still nebulous.
And check the
All Music Guide for some pointers, they're usually pretty good with genres. Although for the most part, I wouldn't want to use any Internet resources for a college project unless they belonged to a university or official scholarly organization.