Octopus Rotary Encoder Brick OBEncoder

Octopus Electronic Bricks, you can use them build electronics projects just as easy as piling bricks. By using Octopus electronic bricks, you may connect Arduino/Freaduino compatible boards easily with various digital, analog and I2C/Uart interfaces. These the breadboard-less firm connection are prepared to extensive modules like poteniometers, sensors, relays, servos even buttons, just plug and play. 

A rotary or "shaft" encoder is an angular measuring device. It is used to precisely measure rotation of motors or to create wheel controllers (knobs) that can turn infinitely (with no end stop like a potentiometer has). Some of them are also equipped with a pushbutton when you press on the axis (like the ones used for navigation on many music controllers). They come in all kinds of resolutions, from maybe 16 to at least 1024 steps per revolution, and cost from 2 to  200 EUR or so.