Black Holes are caused by a very large amount of matter occupying a very small area of space. The concentrated gravity is so high that even light cannot escape from it, hence the name 'Black Hole'.
Black Holes were predicted by Einstein's General Theory of Relativity. This showed that when a star at least three times the mass of our sun dies and collapses there would be a very small core of matter dense enough to form a gravitational field large enough to capture anything within a specific distance from the center, called the Schwarzschild radius, including light. The point at the Schwarzschild radius is known as the Event Horizon.
Within the Schwarzschild radius, past the Event Horizon, the escape velocity, the minimum velocity an object has to travel at to escape an object's gravitational field, is greater than the speed of light.