ishkur’s guide to electronic music v2.0 is online. It’s a Flash presentation with 100+ genres well mapped out according to stylistic affinities as well as chronological “development”. You have to question the linear evolution model, but once you get past the long-winded intro animation, it’s all good fun, and there are multiple audio examples for each genre. Definitely worth a look, whether to learn the basics or, if you are a connoisseur already, simply to fly into an apoplectic rage at everything you think is just soooo wrong.