A black hole is a theoretical region of space in which the gravitational field is so powerful that nothing, not even electromagnetic radiation (e.g.visible light), can escape its pull after having fallen past its event horizon.In simple words ,a black hole is like a vaccum cleaner...but different.. 
when you use a vaccum cleaner, you will see the dirt, and crumbs start to move towards the vacuum cleaner. A black hole is similar to a vacuum cleaner,cleaning up debris left behind in outer space.(this is just a simple defintion to give you the picture....it's not actually like a vaccum cleaner at all as you will see later in this post)
But,it is not suction power that makes things fall into a black hole. Suction would not be strong enough. Instead a black hole uses the power of gravity to pull things towards it. Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may
leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself .
The star eventually collapses to the point of zero volume and infinite density, creating what is known as a "singularity ". (Singularity is a term often used to refer to the center of a black hole, where the curvature of spacetime is maximal. At the singularity, the gravitational tides diverge; no solid object can even theoretically survive hitting the singularity).
As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped in
to an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.
But contrary to popular myth,and the simple defintion that i gave earlier, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the earth's orbit around the Sun would be unchanged. (Of course the Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us!we probably wont survive if this happens) To be "sucked" into a black hole, one has
to cross inside the Schwarzchild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
A common question is that "If we can't see them then, how do know that they are there?"
Since black holes are small (only a few to a few tens of kilometers in size{this may sound big but it is really small when you consider the whole universe}), and light that would allow us to see them cannot escape,thus, seeing a black hole floating alone in space would be hard, if not impossible, to see.
However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accreate matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heatingionises the atoms, and when the atoms reach a few million degrees Kelvin, they emit X-Rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.
Some of you must have heard about wormholes and wondered what it is.....If you ever dreamt of travelling through space in a wormhole,then im sorry to dissapoint you...but as far as i know, worm holes are more science fiction than they are science fact.
A wormhole, in theory, is an opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat - the throat, or passageway, is called an Einstein-Rosen bridge. It has never been proved that worm holes exist and there is no experimental evidence for them, but it is fun to think about the possibilities their existence might create. so keep dreaming...you may just find that travelling through wormholes is possible...
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