The user interface that made the iPod popular. Originally using an actual rotating wheel, the wheel was later made touch sensitive, and its rotation was simulated. Buttons at the top, bottom, sides and center of the wheel select menus and functions. See iPod classic and scroll mouse.
The Click Wheel Hangs On
The shuffle (bottom left) is the only iPod that uses the click wheel interface. The nano and touch (bottom middle and right) migrated to touch screens, and the classic iPods (top), which always had the click wheel, were discontinued in 2014. (Images courtesy of Apple Inc.)