A motor is an output device that results in rotation when power is applied. An actuator is an output device, which is any device that creates movements from an electrical signal. Examples of actuators are solenoids, valves, and pneumatic rams. Motors are a subset of actuators too.
A sensor is a device that provides input to a robot, allowing it to sense its environment. There are more sensor types than a single book can list, so we'll keep to the commonly available and fun-to-use ones.