Rotary encoders can be used similarly to potentiometers. The difference being that an encoder has full rotation without limits (It just goes round and round). They output gray code so that you can tell how much and in which direction the encoder has been turned. They're great for navigating menu screens and things like that.
This encoder is especially cool because it has a common anode RGB LED built in, as well as a push-button. This version has an updated material that is heat resistant and slightly changed dimensions, but should still work well with the Sparkfun rotary encoder breakout board.