These boats are probably the easiest to relate to for the public. Displacements are, from a distance, the closest resemblance to your average water ski boat. With the engine mounted "inboard", usually at the rear of the boat.
Obviously these finely tuned racing boats are a little different than your average ski boat. Construction materials range from fibreglass & plywood through to the high-tech materials of kevlar & carbon fibre, making these craft very light and very strong in relation to your average ski boat.
The term "displacement" is derived from the action of the boat "displacing" the water (pushing the water out of the way) as it moves through the water.
Displacement classes range from 3.4 litre skiffs all the way up to the monstrous Blown Alcohol Displacements.