Look at your "Edit" button. Right next to it is an icon that shows a guy's head. Click it and you can send good or bad rep(utation) points to people. If you think their post was helpful, funny, whatever, or if you just want to be nice about it click that person's rep button and fire away.
On the flipside of things you can give bad rep which is very fun. Especially if you act like a n00b doing it and you don't leave your name and the recipient makes a thread bitching about people not leaving their names on bad rep comments. Muahahhaah.