Cosmic DJ presents itself on the "game" side of things and, based on the limited amount of variety you can actually affect, it belongs there, but unlike Guitar Hero or similar games, you can actually set the rhythm and progressions to be what you want. So, you actually are participating in the creation of the music, not simply determining whether or not it will sound good by playing a rhythm game.
Further, there are two different ways to lay down a track: sequencing it on a grid or using the keyboard like an electronic drum pad, of sorts, to tap out notes at the time in the music where you want that sound to occur. While making a mistake using the Tap method can make music that sounds different from what you were trying for, you can remove unwanted notes when in the Grid Mode, allowing you to correct your mistakes, so you're under much less pressure than, say Guitar Hero or Rock Band.