Most all of these simple mechanical switch encoders are of the incremental type. Here is one typical one:
I played with one for awhile but was not too pleased with it for the following reasons:
Pretty bad contact bounce that has to be dealt with either in software or external components. It limits how fast you can turn the knob also without missing steps, etc.
Most have a mechanical detent that makes a clicking sound as you turn the knob. Lots of times it seemed that the mechanical detent steps didn't line up well with the electrical phasing, it just didn't feel precise in it's mechanical detents Vs electrical steps.
I eventually found a surplus optical rotary encoders for a real bargain price, they sell for $50 new. These are really precise and feel great and no cheap clicking sound. Sometimes E-bay has some good deals on surplus optical encoders, but it is hit and miss.
Anyway whatever you get you just have to play around some with it and the code to get a good understanding. They are cool and make a great user interface. Some have a extra switch on the knob (and extra contacts) that you can press in, so it's like spin to select menu item, press knob in to select item, cool.