Camshafts are located in either the cylinder head or engine block. Camshafts located in the cylinder head are generally know as OHC engines or DOHC (Double overhead camshaft) if it is a twin cam engine. Pushrod or OHV (Overhead valve) engines have the camshaft sat underneath the valve train.
Camshafts are used to manipulate the amount the engine valve travels into the combustion area and also the length of time it is open. This is set by the engine manufacturer . Race engine developers may require a different lift or timing if they made alterations elsewhere in the engine to increase torque or BHP. The cams are nomally adjusted to perfor better at higher revs where the engine will running whilst in a race environment. This will probabaly cause rough running at idle or lower revs.
Camshafts are generally made from chilled cast iron or steel. Some cam lobes are "Sweated" onto the shaft. The principle is that the lobes are an "interference" or "press" fit. In the event of engine failure these lobes can be pushed round on the shaft without being noticed. The 1.7 16v CDTi Vauxhall / Isuzu engine is known for this to happen if the engine timing is forced out of sequence.