I don't profess to be an expert, but usually the term "actuator" typically refers to a device that provides linear motion... like a piston, a rod is pushed in a linear manner when voltage is applied.
A motor is a device that provides rotational movement (like in a toy car, the motor spins a wheel (usually through gears to reduce the speed and increase the tourque).
A servo is a motor along with electronics that provides a feedback loop to the motor. A Servo is a small device that has an output shaft. This shaft can be positioned to specific angular positions by sending the servo a coded signal. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes. In practice, servos are used in radio controlled airplanes to position control surfaces like the elevators and rudders. They are also used in radio controlled cars, puppets, and of course, robots.