The Reactor is a power-producing building. Just like windmills, reactors have to be found individually as tokens, rather than being placed from a tool. Directing a power beam towards a reactor will cause an explosion, making them useful to the disenfranchised griefer types. That said, although this explosion destroys the reactor and causes minor terrain damage, it doesn't destroy nearby Monoliths, so it's unlikely to be too disasterous.
A Power Reactor will create twice the amount of power as a windmill.
The advantage of the reactor over the windmill is that the power does not turn off at dusk and dawn, making these buildings important for powering critical structures like the forcefield and turrets.