Black holes are regions (not specifically objects) in space in which the gravitational force is so strong that nothing, not even light can escape from it. Black holes are so strong that even time is distorted near the event horizon of the black hole. It is virtually impossible to see a black hole because; it can even absorb light thus rendering itself invisible. However, it is possible to see the effects of the black hole in its direct vicinity that is to say the area surrounding it. Black holes were first proposed by Karl Schwarzschild
after Albert Einstein (a brilliant German physicist) proposed his theory of general relativity that proves that gravity can influence light to a certain degree. Schwarzschild gave the solution for a point mass and a spherical mass, thus proving that black holes could theoretically exist.
A black hole is born when a giant star- bigger than a main sequence star (like our very own sun) - explodes in a violent explosion known as a supernova. When it explodes, the supernova leaves behind a neutron star and when the mass of the neutron star has more mass than that of five main sequence stars its gravity will be so strong that it will create a black hole out of itself.
According to Einstein's theory of relativity, any object that has mass distorts spacetime as a metal ball distorts a rubber trampoline. This shows that gravity is actually a manifestation of the curving and distortion of spacetime. For example, a star will distort
spacetime by a whole lot so the gravitational field is bigger thus other masses will be attracted to it and start orbiting around it. However, in the case of a black hole, spacetime is distorted so much that it actually causes a gaping chasm in that direct vicinity of spacetime. A black hole has what is called an event horizon, outside the event horizon matter is not affected by the black hole, however, when we come closer to the black hole's event horizon we start to feel a definite tug but not a pull into the black hole such as when a star changes the path of an oncoming asteroid but does not capture it in its gravitational field. When an object reaches inside the event horizon however, there is no turning back, the object will either be immediately flung into the black hole or go around it a few times before being sucked in, this all depends on the direction the object was heading in the first place.
Basically, there are four main types of black holes, the first being stellar mass black holes which are by far the most common (relatively). These black holes are caused by the gravitational collapse of a massive star as described above. The second type of black hole is an intermediate-mass black hole, these black holes are by far the rarest and not much is known on how they are formed. There is a theory however, to the formation of these intermediate mass black holes, which is the collision of stellar mass black holes and other compact objects. The third type of black hole and most magnificent type is the Supermassive black hole; these black holes are formed by the intense condition at the center of galaxies with high pressure and velocity. These black holes do not take in all the matter they find instead, some is blown as jets- out of the direct center of the black hole, which is perpendicular to the position of the accretion disk (a disk of gas and dust). The final type of black hole is the micro black hole, which are at current more theory than fact.
Finally, black holes emit a certain type of radiation known as hawking radiation which imply that black holes lose mass to this radiation and therefore will evaporate and cease to exist.