Seriously I don't know. I've heard the term many times but I'm still not sure what it means. I think people use it for various meanings. Is there a main definition for it though?
Most Helpful Girl
A hook up is usually casual sex with someone. That's how I read it anyway.
Others use it to describe people who are seeing each other as in "*** and *** hooked up recently". Sometimes hooking up can be used to describe two people who are kissing/groping each other at a party.1