This is a great - but long article. Explains why cats always go for the one in the group that don't like cats! http://www.messybeast.com/cat_talk2.htm
Has pictures explaining ears, tails, body postures... and discusses eyes & etc.
This is an OK link:http://www.best-cat-art.com/cat-body-language.html
With multiple cats in our home, we learned about ears and tails pretty quickly. I do remember the first time our first cat sat in front of the window with his teeth chattering and his tail twitching - we thought he was possessed or something!
Of course, now we know he was just interested, and in "get the prey" mode.
Tails puff during play as well as in true aggression - so it's important to put it all together with the posture, the ears - and the context.