The reciprocating motion of a typical valve, technically a poppet valve, is brought about by its interaction with a cam, an eccentric lobe on a rotating shaft. This interaction may be direct, with an intermediate tappet, or through a rocker arm or a combination of rocker and pushrod. The rotating shaft may be alone atop the head ("sohc," as in single overhead camshaft) or paired ("dohc," double overhead camshafts) or down in the block (teamed today with "ohv,"overhead valves). Each camshaft may be belt-, chain- or gear-driven by the engine's crankshaft. In a 4-stroke engine, its rotational speed is half that of the crankshaft's rpm.
A given contour of camshaft lobe determines a unique reciprocation of its valve. The sketch below shows typical intake and exhaust valve behavior during one revolution of their camshaft, equivalently, two revs of the crankshaft. Viewed from left to right, it begins with the expansion stroke, then exhaust, intake and compression. Measured relative to crankshaft rotation, each valve will display particular timing (when it opens and closes). It'll have a specific duration (the degree it stays open). And it'll have a unique lift (yep, you guessed what lift means). Last, intake and exhaust valves will have a certain overlap (the period when they're both open).
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