It's a high school dance where you ask a person you love very dearly to go with you. It's customary for the guys to ask their lovers to prom in a very thoughtful and creative way, almost like asking someone to marry you. It's a pretty big deal, and some groups in school will rent a limo bus or sometimes just a limo and will all be picked up and driven to the dance. You dress very nicely and you get to dance and have fun with your love. It's the stereotype that afterwards people will go home, get drunk and have sex. That's where the term dumpster baby comes from because sometimes the girl will get pregnant and she doesn't want to take care of the baby. So she just throws it in the dumpster. Terrible but it's more of a joke than anything, but the stereotype is not. It's very much true, and I'd venture to say that at least a fifth (maybe a quarter) of everyone that goes to prom has sex afterwards. There's not much social structure since everyone is really just having fun with their friends and dates. It represents your last year in high school, and it's basically a farewell to all of the good times you've had throughout the past 4 years (it's a senior dance mainly).