black hole is a very very dense singularity ie,1000 earths packed into a very small space.due to this the gravitational force exerted by this singularity is very strong that light cannot pass through it.
they are formed when a massive star dyes ,or due to a massive supernova explosion.these stars eventually end their lives by an explosion.
event horizon in astronomy means a point where there is no return.
in black hole ,when an object enters a black hole a certain point is reached when there can be no return .that is why we cannot see a black hole,the light traveling near it gets sucked up ,so it stays invisible until some object gets absorbed.
i hope this helps u...