basically, when you help me or even just someone else out or just make a post that i thoroughly enjoyed reading, whether it was a good story, made me laugh, or maybe if i got into an interesting and informed debate with you that didn't just end with "well fuck you" or something, then i'll give you karma, because you are a worthwhile member who is in my opinion, contributing well to the site. If you aren't helpful at all, blame other people, or just do stupid shit in general, i'll give you no karma. And even though it's died down now, saying full tilts unless it was 100% the funniest situation ever got you bad karma.
unfortunately, the way it really works for most people here is more like "hey, this kid disagrees with me about the most meaningless trivial thing ever....MINUS KARMA!!!!"