Rotary encoder

A rotary encoder is a device that can keep track of how much something has turned. This is called its angular position. That information is useful to know how far a motor has turned, for example. 
Rotary encoders are often used in manufacturing and robotics. This is because they need things to be measured and controlled very accurately.
Basic types.
Absolute.
An absolute encoder always keeps track of what value its on, even if it isn't powered.
Incremental.
An incremental encoder only keeps track of changes. It gives off signals if it is turned one way or another.