Reaktor is a puzzle game that works under the premise of combining three objects of the same color to clear them off of the screen. Your motivation for doing this is to keep yourself from dying, which will happen when the screen fills up with too many of these objects.
Reaktor, however, does not opt for the simple top down accumulation of colored items; instead, the playing field is circular and each pizza slice shaped part of the circle fills up with its own set of items.
The story laid on top of the gameplay is trivial; each planet is in danger of being destroyed by a nuclear meltdown. The blocks that you are destroying are plutonium, and when the plutonium becomes too much, you lose. To make things worse, if you wait too long to place a plutonium block yourself, a random one will shoot out automatically.
At first glance, Reaktor seems to have something new going for it. Closer inspection will reveal that the game sticks to the same formula as almost every other puzzle game out there. This is both good and bad. While it's good to know that the formula still works, it doesn't do much in the way of innovation.
Reaktor's strict adherance to the puzzle formula is apparent in its powerups and special blocks as well. A power meter fills up as you destroy plutonium, and when it becomes full, you can unleash it in all its glory to destroy almost half of the board. There are also special plutonium blocks that will destroy several sections of the board in one go.