Minecraft wiki entry for Redstone_mechanics, associated text: A power component provides power to other parts of a circuit—e.g., redstone torches, buttons, levers, redstone blocks, target blocks, etc. Some of these fall into one of three overlapping subgroups:
Switches provide power depending on request by the player. Buttons and levers are switches.
Sensors provide power or signals (see below) in response to some environmental condition. Pressure plates and Observers are sensors, and comparators can be used as sensors. Note that some pressure plates can be triggered by a player standing on them, which also qualifies them as switches.
Logic components provide power conditionally, depending on their input conditions. Redstone torches, and comparators are classic logic components; redstone wire and ordinary opaque blocks can also be used to combine signals in various ways.
A transmission component passes power from one part of the circuit to another. Redstone dust (placed as redstone wire) is the most fundamental transmission component, but redstone repeaters and redstone comparators are also important.
A mechanism component affects the environment (by moving, producing light, etc.)—e.g., Doors, pistons, redstone lamps, dispensers, etc.