Based on the age range and context this term is put in, it can mean a number of different things
1) Middle school age- kissing or making out at most (unless for those few mature sluts who make make it farther)
2) High School- Generally having sex. For most freshman and some sophomores, this means kissing or making out, unless they make it farther which can be common
3) Young adults- sex.
4) Older adults, like parents- Being really nicely helped out
1)John and Lilly are hooking up at the school dance tonight!
2) I hooked up with the quarterback, his d is tiny
3) I think I might've hooked up with Jerry when I was drunk.....
4) Wow!! Bobby really is hooking me up with those concert tickets!
John hooked up with Mary. (so what did they actually do? Did they meet at the park? Did they talk? or did they duke like viagra-injected rabbits? who knows...)
any form of getting some type of action, such as, making out, feeling things, or oral pleasure or sex between 2 people. "A hook up" is whats referred to as doing those things when your not going out with anyone
Guy:"yo, i hooked up with kristin last night!"
Guy 2:"Nice, what did you do?"
Guy1:"we only got to third"
Casual sexual activity. Sex while not in a relationship with eachother. Similar to a "one-night stand," but it can happen repetedly. As long as the two people are not serious about eachother, and they have sex, it's hooking up.