having a rapping contest against someone else in front of an audience, where most of the lyrics are insults directed at the other rapper which have to rhyme at the same time.
1 rapper goes first, then after he's done its the other persons turn and then someone decides whos was better, or after the second rapper has finished the first one can go again if he has more to say.
if one of the rappers suck at rapping they can get boo'd off stage by the audience. (they usually let some of the audience come up and take the mic for a few minutes at the end)
watch 8 mile to see a rap battle