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 a digital code. We can use this information to tune a radio, set volume, etc.
This page deals with the use of the output of an incremental encoder which provides information about the motion of the encoder shaft - CW or CCW.
An incremental rotary encoder provides coded outputs (only) when the encoder shaft is rotated. This page deals with use of a mechanical encoder (switch closures) which may need a form of debouncing to provide accurate information. They employ two outputs called A & B, which are called quadrature outputs, as they are 90 degrees out of phase, resulting in a sequence of closures referred to as Grey Code.