The basic gameplay has you following a series of discs around the iPhone's screen, and you tap the screen to create matching orbs for them to touch. Touch to create an orb, then line it up so the bouncing disc touches it at the right time. The twist is that the discs are all different colors, and the orbs you create change colors over time. For example, a white disc simply has to hit a white orb, which is the color it is when it's first created. But a red disc can only hit an orb after it's turned red -- about a half second after you create it. You have to calculate the angles and time your orb placement just right in order to match colors and clear the discs.
It sounds complicated, but in practice, it's not. What you end up with is a fun mix of a timing and angle game that's pretty addictive. There is also a "preschool" mode for kids (much less complicated -- just tap the moving objects to clear them), and there are a number of different difficulties and powerups to contend with. OpenFeint integration extends replayability, and the team is even offering a monthly contest with cash prizes for the game's best players. Tap Disc is definitely worth checking out.