Minecraft wiki entry for Redstone_mechanics, associated text: Many types only perform an action when initially activated by a rising edge (command blocks execute a command, droppers and dispensers eject an item, note blocks play a sound) and won't do anything again until deactivated and then activated again
Other mechanism components change their state when activated, and then change back when the activation ends; Redstone lamps stay on while the power continues, while hoppers stay disabled. Pistons will extend when powered, and retract when the power turns off. Doors, fence gates, and trapdoors will open on a rising edge, and close on a falling edge; however, most of these (not iron doors or iron trapdoors) can also be opened or closed by players regardless of the redstone power. If they were already open when power turns on, or closed when power ends, they will simply remain so until their input changes again.