What are the main genres of electronic music, and what's the difference between them?

There are more popular genres than others,  but there are no main genres.  There are as many genres as there is our propensity to classify and label things ^_^

The differences are what we choose to pickup on.

Now if you would like a list,  look no further than a wiki

http://en.m.wikipedia.org/wiki/List_of_popular_music_genres