Here are a few types of online poker players that you may want to keep an eye out for:
Look for people losing a really bad hand and then betting big the very next time they get dealt cards, and players expressing their anger in the chat box. If they're bothered enough to type ANGRY things, you can bet they're really RAGING. And if they're RAGING they're ripe for the taking. Don't waste time replying if they insult you. Play carefully and hit them hard when you get good cards - ignoring them and then taking even more of their chips is guaranteed to wind them up big time.
Some people spend more time typing into the chat boxes than they do playing their cards. Try it for yourself - if you're feverishly typing great witticisms into the box it takes time and, before you know it, the action is round to you again and you're forced into making a snap decision. Watch out for the talkers and lean on them. And try not to get drawn into lengthy conversations yourself. You're not there to make friends.
Maniacs love throwing their chips around for the sheer hell of it, and you'll find more playing poker online than anywhere else. Join a sit-and-go online and more than not you'll see someone moving all-in pre-flop, with only the blinds to pick up. It's terrible play - if the maniacs got a strong hand then he's not going to get any action. If he's got a weak hand he's risking everything for the sake of a few chips. Don't get drawn into any unnecessary confrontations, but when you get a monster, hit him hard. Make sure that you're the one he gives his chips to.