A black hole is a place in space where there is a huge amount of mass in a very small area. Because gravity is the same as mass times 1/volume, when you have a lot of mass in a small volume it makes so much gravity that even light can't get away from the black hole, so it looks black to us. That happens when the speed you would have to be going to overcome the gravity and get away from the black hole is bigger than the speed of light.
If you think of space and time the way physicists do, as being both part of one thing, spacetime, then a black hole might be a place where spacetime bends in a special way so that when things near the black hole go forward in time, as everything does, they also have to go forward in space and deeper into the black hole.
Astronomers think most black holes are about the size of large stars, and get started after a supernova when big stars collapse in on themselves. There may also be a lot of teeny-tiny black holes, called micro black holes, but nobody has seen one or proved that they must exist. These micro black holes might have formed during the Big Bang.
Many astronomers think that there are black holes at the center of galaxies of stars, including our galaxy, the Milky Way. These would be extra-big black holes, millions or billions of times the mass of our Sun. Nobody's ever seen a black hole, so the only way we know they exist is that we need them to make calculations about space come out according to the laws of physics.
Because stars spin around, black holes made from old stars probably also spin around. Probably these black holes spin very fast, because they are so much smaller than the stars they came from, just as an ice skater or ballet dancer spins faster when she pulls her arms tight in to her sides, because of conservation of momentum.
To find out more about black holes, check out these books from Amazon or from your library: