A timer or countdown timer is a type of clock that starts from a specified time duration and stops when reaching 00:00. A simple timer is an hourglass. Commonly, a timer triggers an alarm when it ends. A timer can be implemented through hardware or software.
Stopwatches operate in the opposite direction, upwards from 00:00, measuring elapsed time since a given time instant.
Time switches are timers that control an electric switch.
A typical kitchen timer
Image: Electromechanicaltimer
Clockwork refers to the inner workings of either mechanical devices called clocks and watches or other mechanisms that work similarly, using a series of gears driven by a spring or weight.
Clockwork of mechanical Prim wrist watch
Keys of various sizes for winding up mainsprings on clocks
Mechanism of a Wall Clock, Ansonia Co. 1904
Movement of a grandfather clock with striking mechanism