Except for rotaries engines, all four-stroke engines use some sort of camshaft. They are widely considered the "brains" of an engine.
A camshaft's main job is to open and close the engine valves, via the lifters and rocker arms, in time to the piston's position in the cylinder. This causes the engine to take in air, burn fuel and expel exhaust gasses.
The camshaft spins at twice the sped of the crankshaft. The camshaft's lobes push on the lifters and pushrods, which move the rocker arms and open the valves.
Camshafts come in four varieties: flat-tappet, roller-tappet, hydraulic-lifter and solid-lifter. Most engines come from the factory with hydraulic lifter, flat-tappet camshafts.
Roller camshafts are designed for use with special lifters. These lifters have a wheel on the bottom that helps them follow the camshaft's profile more closely. This allows the maker to grind the camshaft so that it opens and closes the valves faster and keeps them open longer.
The lifters for most camshafts use engine oil pressure to cause them to open the piston more at higher revolutions per minute (RPM). Solid lifters don't vary with RPM.
