The starter is an electric motor which uses power from the battery to crank the engine.
Current flow to the starter is controlled by a starter relay or solenoid.
Starter relays are usually mounted on the inner fender near the battery and are used only to control the current flow to the starter motor.
Starter solenoids are mounted directly on the starter and are used to control current flow as well as starter drive engagement.
Most vehicle use only a relay or a solenoid although it is possible to use both.