A Black Holes is frequently thought of as some sort of mysterious phenomenon that acts like a "vacuum cleaner in space", sucking in and destroying anything in its path. This view is partially correct and partially erroneous. While it is true that once something is pulled into a Black Hole it cannot escape, the notion that black holes automatically consume everything around them is certainly not accurate.
To understand Black Holes, it is helpful to first understand where these astronomical oddities originate. During a star's normal life, it is balanced by two opposing forces: gravity and nuclear pressure. Gravity acts on all mass, pulling it inward toward a point. However, a star is continually fusing elements into heavier elements (normally Hydrogen into Helium, during most of its life). This fusion produces energy, in the form of light. This light is thrown off in all directions from the center of the Sun, and literally pushes outward on the gas of which the Sun is composed.
The outward pressure causes the material making up the Sun to maintain a balance between gravity trying to pull it inward, and nuclear pressure trying to push it outward. This balance can be maintained for most of the star's life, until it is finished burning through its fuel. Once all of the Hydrogen in a star is gone, the star will start to collapse on itself, since gravity is still acting but there is no more Hydrogen to create nuclear pressure. This will cause the core of the star to attempt to fuse Helium in the star into heavier elements. The results of this attempt will depend on how much material (mass) the star has. The more material it has, the easier it is for a star to produce the intense pressure needed to get heavier elements. But no matter what, the heaviest element the star can get to is iron. It takes more energy to get iron to fuse into something heavier than the energy the Star gets out of the fusion reaction. So now, with nowhere else to go, what happens to the star when it collapses?
There are a few different possibilities for what happens to the star at this point, and they depend entirely upon the mass of the star. If the star has more than three "solar masses" (meaning it is more than 3 times bigger than our Sun) at the time of its collapse, it will become a Black Hole.
The star will start to collapse under its own weight. When it does, the protons and neutrons that make up the elements of the star will collapse onto themselves and become like a big ball of neutrons. If the reaction stopped here, it would be what is termed a "neutron star". However, gravity is so strong that it is not possible for even the neutron star to sustain itself, and collapse goes even further, until the point that all of the matter which made up the star is collapsed to a single point – a Black Hole. This "Black Hole" is named this way because once something gets too close to the hole, it can't escape – even light. Therefore, any light that falls onto the Black Hole is trapped forever inside the singularity. This "point of no return", where light cannot get out, is called an "Event Horizon". This is not surprising, however, since every gravitational body has what is called an "escape velocity". This is the speed someone would need to travel to get away from the body. For example, the escape velocity of the Earth is 11.2 kilometers per second. Anything going faster than that can get away from the Earth. Since light goes at around 300,000 kilometers per second, it naturally has no problem getting away from the Earth. However, the more mass an object has in a compressed space, the higher the escape velocity is. A Black Hole has so much mass pressed into such a tiny volume (technically it is compressed to a zero volume) that the escape velocity exceeds even the speed of light.
Understanding a complex phenomenon like a Black Hole is helped by first understanding how one would form. This knowledge helps us to recognize that while Black Holes are oddities, they are still a natural phenomenon waiting for us to explore them.