An arcade game originating from Japan which features two dancing surfaces. The idea is to hit one of or a combination of arrows (up, down, left, right) as they are displayed on the screen in time with the music which is being played. As you move up in difficulty, the movements become more complex which in turn makes the player looks as if they were executing a form of dance. Definitely a public game, and best played with another player. It's harder than you think.
You can find some DDR steppings at http://www.ddr.sh.
Some of the easier songs to dance to are:
Have You Never Been Mellow
Boom Boom Dollar