Rotary encoders are useful as rotation sensors or selectors and look similar to potentiometers. However they are not like potentiometers at all, so it's important to realize the difference! These rotary encoders rotate all the way around continuously, and are divided up into 22 'segments'. Each segment has a click-y feeling to it, and each movement clockwise or counter-clockwise causes the two switches to open and close. There is no way to know what the current 'position' is - instead you would use a microcontroller to count how many 'clicks' left or right it has been turned. If you need to detect rotational 'position' a potentiometer would be a better choice.

Tutorial from Instructables.com 

Specification:

Working voltage: 5V
Material: PCB + Brass
Weight: 10g
Size: 32 x 19 x 30mm