An actuator is the component of a machine that produces a force, torque or displacement when it is supplied with an electrical, pneumatic or hydraulic input signal. It is thus the "moving" counterpart to the sensor: the sensor perceives, the actuator turns control commands into physical motion.
In robotics, actuators sit at the joints and convert energy into motion. The three most common technologies are electric drives (servo and stepper motors, precise and easy to control), hydraulic drives (very high forces) and pneumatic drives (fast and inexpensive, but harder to control precisely).
The number, arrangement and power of the actuators largely determine a robot's degrees of freedom and payload. Every actuator requires a control signal and an energy source. It can be likened to a muscle that enables the movement of arms or legs.