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.
News: All of Octopus bricks has supported Scratch programming. ElecFreaks builded EFBlock support drag and drop programming . Guide please check here