Black holes

Black holes, apart from being a favorite topic of sci-fi writers, are physically very cool objects. The basic principle is that it's a blob with an escape velocity so high that even light can't escape it. In other words, since no information can travel faster than the speed of light, we cannot possibly know anything about the interior of a black hole. The 'interior' is determined by what's called the Schwarzschild radius, or the event horizon. This is the point of no return - once you hit that point, whether you're a person or a bit of information, you're in the black hole and not coming out.
A fascinating consequence of the lack of information exiting a black hole is what's called the 'no hair theorem', which says that black holes have only three observable properties: mass, angular momentum, and charge. Since any sort of charge imbalance out in space inevitably balances out very quickly, we can ignore that case. So black holes really just have mass and angular momentum.

Oh, and another interesting fact about them:
 Nonrotating black holes have this thing called a photon sphere, around fifty percent further away from their centers than the event horizon, at which photons can orbit the black hole at the speed of light. That's a direct result of general relativity, which states that mass and energy bend spacetime. Light takes the straight path in this curved space time, so heavy objects can noticeably bend light. Imagine that, an orbiting photon!