Black hole:

These mysterious monsters suck in anything that gets close to them. They are very small and hard to see, but are EXTREMELY heavy. They have more gravity than any other single object in the universe. They are created when big stars collapse and explode. The stuff left over after the explosion is the black hole. It's black because not even light, the fastest thing in the universe, can escape it. The result is that it looks black.

The event horizon is the border of the black hole. Once you cross this line, there is no excaping it's gravity!