They occur when dry air blows into a thunderstorm, evaporating some of its rain, which in turn, cools the air. That air, now much colder and heavier than the air around it, sinks like a rock, out the base of the thunderstorm, and rushing toward the ground. The rushing air can bring roaring wind speeds of up to 150 mph. It can also bring an incredibly intense rainfall along with it.
To a person standing on the ground, it'd be as if the cloud dropped a huge water balloon on your head, only it'd be filled with cold, dense air.
Damage from stronger microbursts can sometimes rival that of tornadoes.