There are five basic sections of a watch.

1. The mainspring, and its winding mechanism, which provides power..
2. The train, which consists of gears, wheels & pinions that turn the hands.
3. The escapement consisting of the escape wheel and balance that regulates or controls.
4. The dials and hands that tell the time & setting mechanism.
5. The housing consisting of the case and plates that protect.

A watch is a machine with a power source that drives the escapement through a train of gears, and it has a subsidiary train to drive a hand.

The motion of the balance serves the watch the same as a pendulum serves a clock. The balance wheel and roller oscillate in each direction moving the fork and lever by means of a roller jewel or pin. As the lever moves back and forth it allows the escape wheel to unlock at even intervals to insure equal time through the 24 hours and causes the train of gears to move in one direction under the power of the mainspring.

Thus, the mainspring is allowed to be let down or unwind one pulse at a time. (The escape wheel moves in one direction similar to a turnstile.)