A multi-barrel machine gun. It's unwieldy, but once it starts firing it fires very fast. Where most self-loading guns are powered by the energy from the gunpowder, the minigun uses an electric motor to rapidly cycle cartridges through the weapon.
Heavy weapon that fires a large barrage of bullets after a long warmup time.
Miniguns are excellent crowd control weapons. Their miss radius and extremely high DPS are valuable assets when it comes to crowd control, especially in later game when there are many more enemies in raids. If you manage to get multiple miniguns to your name, you could possibly have a great deal of firepower on your side, but also a high risk of collateral damage if within your base, dependent on setup.
The minigun is even more powerful if equipped on trigger-happy colonists, as they have a 50% reduced warm-up time, allowing them to fire much faster; the accuracy penalty is completely irrelevant for this weapon as it has a forced miss radius. This won't change whether the minigun is worn-out and of awful quality, or pristine and legendary; quality also bears absolutely no relevance to miniguns unless at point-blank, and lower quality will actually be better because they won't affect colony wealth as much.
However, for the same reason it may be impractical to use outside of point-blank to very close range combat (even with higher qualities), or against single or highly spread out targets.
A single centipede armed with a minigun is almost trivial to deal with if engaged at even medium range and behind a wall or sandbags, simply spreading out sufficiently, or distracted with lone colonists; stack up multiple miniguns with Inferno cannons and Heavy charge blasters and things can quickly get out of hand without killzoning, (ideally at least 6) mortars or just outright raw firepower.
The minigun is a good weapon against entrenched raiders packed closely together. Due to its radius and high DPS it can hit multiple raiders at once, rapidly pumping their bodies full of lead.
It often requires manual targeting as colonists tend not to choose the best targets for the minigun to shoot at. It also performs significantly better if you're firing at a point right behind the enemy by manually aiming it there.
While it is capable of hitting single targets (as of Beta 19), it is not as effective as normal weapons when doing this so it is not recommended.