Rotating black holes (a.k.a. Kerr black holes) have a unique feature: a region outside their outer event horizon called the ergosphere. A black hole is formed when a really massive star runs out of fuel to fuse, blows up, leaving its core to implode under its weight to form a black hole.
The centre of a black hole is a gravitational singularity, a point where the general theory of relativity breaks down, i.e. where its predictions don’t apply. A black hole’s great gravitational pull emerges as if from the singularity.
The event horizon describes a sphere around the singularity: when anything enters this sphere, it can’t escape unless it travels faster than light (which is impossible). Just beyond this sphere, a rotating black hole will also have an ergosphere – a bigger sphere that an object can enter and then leave if it’s moving fast enough, but still less than the speed of light.
