A popular method for
a cappella groups and other such un-
accompanied ensembles to reproduce the sounds normally made by
drums and other
percussion instruments, and hence provide a solid rhythmic backing to a
vocal song. Basically, doing it right means losing all
inhibition, not caring how
stupid you might look, and just trying to sound as much like a drum kit as possible.
The Bloodhound Gang's "
You're Pretty When I'm Drunk" is probably the most common recording on the
popular market that makes use of VP (as it's called within the a cappella
community).
Basic technique involves finding the right nonsense syllables to represent standard components of a drum kit: for example, db for a bass drum, dm at varying pitches for toms of varying sizes, ts for a hi-hat, or ksh for a crash cymbal. Once you master these, tongue tricks are the primary ways of creating new sounds. The foremost consideration, both in practice and performance, should be rhythmic accuracy, but almost as important is controlling your spit -- saliva has a tendency to go flying everywhere until you figure out how to stop it.
When soloing, some people refer to people performing VP as "human beatboxes." This may be true, but there are few finer ways to get me to bitch-slap you than saying that in front of me.