A micrometer is a tool used to measure distances with an accuracy of a thousandth of an inch. Several types exist, with variations on the type of measurement made (e.g. internal, external), on the minimum and maximum measurable distance, and on the system of measurement (e.g. metric, SAE).
A micrometer measures between the rotating spindle and a fixed anvil. The threads along which the spindle rotates is precisely machined to advance the spindle an extremely small distance with each revolution. A barrel rotates with the spindle to indicate the distance between the spindle and the anvil.