They are cylinder shaped objects that look like a small juice can - some are solid and some are hydraulic. They bottom against the cam shaft. As the cam rotates they are forced upward. The upper end of the lifter is attached to the push rods. The upper end of the push rod is attached to the rocker arm. The rocker arm is attached to the valves (intake and exhaust) As the cam rotates it pushes the lifter up, pushing the push rod up, pushing the rocker arm down, opening the appropriate valve to open allowing either fuel to enter the combustine chamber or burned fuel to be exhausted.