some generalizations that may be wrong or right:
electonic music can be broadly classified as:
1. Steady beat
or
2. Broken beat
under steady you have the steady 4/4 kick, as cy said, thats: tug tug tug tug...
here you'll have: house (which has a soulful, earthy quality), techno (which is more minimal, with futuristic/tech themes), trance (which is for people who like to trip out to E at 5am haha- but seriously faster bpm, lots of layers, and "heavenly" vocals)etc etc
under broken you have of course, drum n bass, jungle, breaks, 2 step, dubstep, grime, dancehall, hip-hop etc. here, the kick is, well, broken, eg: tug, pak, k-tug, pak, ktug-tug, pak...
between these two broad groups, there's a lot of cross-pollination (eg big beat, IDM, tech-step, etc etc) and so labels are pretty much meaningless now, and only useful for the sake of convenience in conversation and quick descriptions and for record stores...
if you want a really funny and yet pretty enlightening guide to the different genres, i highly recc'd you check out ISHKUR'S GUIDE TO ELECTRONIC MUSIC. google it now- its a flash application.
