A redstone circuit is a contraption that activates or controls mechanisms. Circuits can act in response to player or entity/mob activation, continuously on a loop, or in response to non-player activity (mob movement, item drops, plant growth, etc). A useful distinction can be made between a circuit performing operations on signals (generating, modifying, combining, etc.), and a mechanism manipulating the environment (moving blocks, opening doors, changing the light level, producing sound…
