The camshaft is a system used in piston engines to operate poppet valves. It consists of a long rod running the length of the cylinder bank with a number of small bumps, the cams, attached to it, one for each valve. The camshaft is geared (or belted) to the crankshaft to turn with it, and the cams force the valves open by pressing on the valve stem as they rotate.
Depending on the location of the camshaft, the cams will operate the valves directly, or through a linkage of pushrods and rockers. Direct operation leads to fewer losses and simpler mechanics, but requires them to be positioned at the top of the cylinders where they are somewhat "in the way". In the past when engines were not as reliable as today this was seen as too much bother, but with the superb reliability of modern engines almost all designs use the overhead cam system, or when there are two, dual overhad cam (DOHC).
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