You see them in various stereo systems and such, they are used to simulate a potentiometer. There is no limit to how much you can turn them in either direction and as you turn them they send pulses indicating the direction. What are they called?
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code.
There are many different varieties and you'll need to further define your requirements to decide which is best for your needs.