Most circuits wait for input. A clock doesn't — it feeds its own output back to its input and oscillates on its own, producing a steady on-off beat you can use to drive anything on a schedule.
The torch clock
The classic clock is a single redstone torch wired back to the block that powers it. The torch powers the block; the powered block switches the torch off; that unpowers the block; which switches the torch back on — forever. Each flip takes one tick, so the torch blinks.
Setting the speed
A raw torch clock is fast and a little twitchy. For slower, steadier timing, build the loop out of repeaters instead and dial in the delay — each repeater adds 1–4 ticks, so a longer loop ticks more slowly. This is how you tune a clock to drive a farm, a light show, or a sequencer at exactly the right pace.
Pulses
Sometimes you want a single blip rather than a continuous beat. A pulse is one brief on signal. Buttons emit one automatically; pulse shapers stretch or shorten signals; and edge detectors fire a pulse the moment a signal turns on (or off) — useful when you want to react to a *change* rather than a steady state.