Jet drives use an engine to power a strong water pump. The pump sucks up water and then forces a jet of water out the back of the vessel to thrust the vessel forward. Directing this jet of water steers the vessel.
PWC are the most common type of vessels that use a jet drive.
Jet drives also may power larger vessels (jet boats) and are used commonly for vessels designed for shallow water conditions. Jet boats can have inboard or outboard jet drives.