There are 3 kinds of rap, but one of these three kinds gives a bad rep for the other two.
Old Skool Rap (A.K.A Hip Hop):
Hip Hop was the rap of the past. Listen to MC Hammer and Vanilla Ice and compare to Modern rap. Hear the common beat? The only main difference is that hip hop is more like dance tunes and has less Swears.
Lite Modern Rap:
Lite modern Rap is a Fusion of R&B, Pop, and Modern Rap. Although it has more swears then Hip hop, It has less then Modern Rap. Nelly and Black Eyed Peas are the two common artist that have Lite Modern Rap in there songs.
Modern Rap (A.K.A. Crap
Modern Rap is basically a bunch of swears going to a beat, with the exception of a few songs. the most Popular Modern rap artists are eminem and 50 Cent.