It consists of 9 rounded-off, bubble-like bottons, spaced evenly apart with 5 on the bottom row, and 4 above those, spaced in between.
There are upwards of 13 different Mixed, numbered from 1-13.
Generally, there is a different theme for each mix, such as the Diner-Themed Pop'n 9, and the Halloween-Themed Pop'13. (Though I may be wrong on 13's theme.)
Many people think it would be terrific if Pop'n Music became a mainstream game, but, look what happened to DDR. Nothing good. Fan-boys screaming at their mommies for the latest version for home consoles.
Pop'n Fan - "Hey, fuck yourself. DDR sucks. Play Pop'n Music.