BIT.TRIP CORE is a pixel rhythm game loosely based off of Missile Command, in which the player must fire in one of four directions in time with music in order to "shoot" bits which are moving through the screen. It is the second game in the BIT.TRIP series, and sadly the only one of them which I wouldn't recommend. I would generally consider it to be among the more difficult BIT.TRIP games, and that is without getting into the imperfections there is with the timing in the PC release.
Like other BIT.TRIP games, as you string together a number of successful hits without a miss, the game will move to a "higher" mode which has more graphic effects, and as you miss enough without getting an appropriate number of successes, you will move to a "lower" mode, which has fewer. CORE starts the pattern of adding a new mode with each successive game, adding SUPER.
For people who really enjoy rhythm games, I'd recommend checking it out, but in another format - it's difficult to play in its current form, albeit not impossible. It's not a *bad* game, just a bit difficult to play.