A black hole is a misnomer. It is neither a hole, nor is it technically black. The primitive name for them, “Dark Stars,” would be technically more accurate, but not as sci-fi sounding. Here’s the best way to think about a black hole:
Picture yourself on the surface of the Earth (should be easy). Now, if you were to toss something up in the air, it would be pulled back down due to gravity. Obviously, if you were to toss it with a large enough initial velocity, it could escape the Earth’s gravity (this is called escape velocity). Now, let’s say the mass of the Earth was just increased to that of Jupiter, but the size is still the same. Now the gravity is inordinately larger, and, in the instant before you become liquefied by it, you notice that the initial velocity of an object would have to be even higher to escape here. Now, you could keep increasing the mass of the planet without increasing the radius, and eventually reach a mass where, at the radius you are at, the escape velocity needed exceeds the speed of light. If you were to shine a light upwards from the surface at that point, it would never be seen outside the planet. You are now inside a black hole (you are also dead).
See, every massive body (read: a body that has mass) in the universe has what is called a Schwarzschild Radius. This is the radius at which the escape velocity of the body is greater than the speed of light. For a relatively small (mass-wise) body like the Earth, that radius is minuscule. However, the technical definition of a black hole is a body whose Schwarzschild Radius is greater than its actual radius—that is, the radius at which light cannot escape is further out than the surface of the body. This radius is also called the black hole’s Event Horizon, the “invisible” line which denotes the beginning of the black hole. The mass itself is called a singularity, a tiny point of insanely huge density. To investigate why this is, we’ll examine how black holes are formed.
When a sufficiently massive body, such as a star much larger than the sun, finally exhausts all its fuel, it can no longer sustain the fusion reactions that keep it stable, and it begins to collapse under its own gravity. Now, stop reading for a second, and drop something on the floor next to you (I do this demonstration a lot in public places. I like dropping things). Did you notice how it fell to the floor, and then stopped? That’s because gravity isn’t really that strong. It’ll pull you down, but the forces that bind together molecules and atomic nuclei are much, much stronger. However, if the star is massive enough, the force of its gravity will break the bonds holding them together. At this point, it may stop, as the gravity isn’t quite strong enough to break down the building blocks of atoms, and the star can become a Neutron Star, which is composed entirely of nuclei on the surface (and, technically, is one big atomic nucleus), and a quark-gluon plasma in the core (complicated stuff that I won’t get into). However, if the gravitational force is still strong enough, it will even break that down, pushing all the mass together as close as possible, into what is called a singularity. Obviously, if the mass is that concentrated, its Schwarzschild Radius will exceed its natural one, and it becomes a black hole.
Two more things now. First of all, a thought experiment: say you have a tiny black hole where you get Earth’s level of gravity at about 1 meter away. What would happen if you were to try to reach out and touch it? Answer: You would be town apart. Because what I haven’t explained yet are tidal forces. See, gravity scales at a factor of 1/r2 (the full formula is F=G(m1m2)/r2). That means that the gravity at different points (even as small as a few centimeters apart) is drastically different (moving one meter closer the the black hole quadruples the gravitational force). The effect of this is that, as you reach out your hand, your hand is feeling twice Earth’s gravity, and your arm is feeling the moon’s gravity. Which kinda screws up your day (and your arm).
Finally, technically, black holes aren’t black. They radiate. “But Austin!” you may cry, “You just said that light can’t escape them!” Indeed I did. But here’s where things get fun. In any system with energy, weird things happens on a quantum level. A subatomic particle and its corresponding antiparticle (such as a quark and an antiquark (quarks make up protons, neutrons, etc.)) can spring into existence, “borrowing” energy from the system to do so (E=mc2). Then they usually collide and annihilate each other, releasing their “borrowed” energy back into the system. However, if this happens on the event horizon of a black hole (“borrowing” gravitational energy from the black hole itself), one of these particles may get pulled into the black hole, and the other may fly off into the universe, which means that the black hole has now lost energy. This process is called Hawking Radiation, after the physicist Stephen Hawking, who is pretty damn cool. You should read his books.