I don't have a chart, but when you think electro, think bumpin', booty-bass style, or some of the more 80's stuff like Kraftwerk and so on. The classic electro beat would be something like 'buh-bu-bu-bu-BUH' vs. the classic 'boomchick boomchick boomchik boomchick' of techno. Techno itself encompasses a fairly wide range of styles, for example classic Detroit techno (slower, almost house-ier sound) to Berlin techno (harder, more bangin'), minimal, acid (303 sounds), etc. There are a lot of different genres and subgenres in electronic music, but it's hard to describe some of them without having you listen to stuff, and aside from that, a lot of times its hard to categorize something in one genre or another. I would advise you not to worry about it too much.
However, if you want to hear some of the differences, I would advise you to listen to some of the Digitally Imported net radio stations, as they seperate them by genre. They have several different trance styles, as well as hardcore and oldskool. For electro, check out Electro Radio, although they don't play pure electro all the time. For house, I recommend Housemusique, and for psytrance/goa, Philosomatika is a great station. I don't know of any stations that play pure, unfiltered techno all the time (unfortunately), but I can send you some good mix sets if you're interested.
Also, keep in mind that the labels can sometimes be confusing - for example, what people in the US call 'hardcore' is totally different from what people in the UK generally refer to as 'hardcore'. Also, many people refer to electronic music in general as 'techno', when in reality techno is its own genre, as I mentioned above.