The term "boom bap" is an onomatopoeia for the the kick "boom" and the snare "bap". This style of hip-hop originated on the east coast in the early 90's. It is usually recognized by an acoustic drum loop/break sampled from a vinyl record that is then chopped up using a sampler, such as the Akai MPC or the E-mu SP1200

check out this article on boom bap's origins.