I find this link to be extremely helpful in educating people about electronic music.
Ishkur's Guide to Electronic Music
For those who appreciate the subtle nuances:
Although it the guide is helpful, there are a number of flaws and inaccuracies. Ishkur also injected his own taste; do don't expect fair representation, though the music samples speak for themselves and generally represent well.
Wikipedia is also an excellent source in that it's peer-reviewed and more inclusive of genres that have emerged recently like dubstep and breakcore. Some older genres and sub-genres are neglected in Ishkur's guide as well - drumfunk being an example - but mentioned elsewhere.
[size=10]Knowing about dance music is so fucking useless...[/h]