Blackholes
Blackholes have fascinated science fiction writers and scientists alike since the term was coined in 1969 by scientist John Wheeler. However, the idea that gravitational anomalies can affect light was first proposed by Newton when discussion the particle properties of light. We now know that the wave/particle dual nature of light makes it possible for Newton's gravity to have an effect on light. If such large gravitational objects do exist, the may trap light thus begging the term "blackhole".
The formation of a gravity well strong enough to pull in light is only formed by masses so big (such as suns), that when they loose all energy to expand outward have so much gravitational pull that they collapse to a single point, infinitely dense. Not all stars will collapse to form blackholes, only those above the Chandrasekhar Limit (a mass much larger than our sun) have enough gravitational attraction to overcome the repelling force of their protons and neutrons that are being squashed together. When this collapse happens, the gravitational energy and mass of the star have nothing stopping them from continuing to pull the star closer together. All of the mass is thought to collapse infinitely small, with a very high gravitational attraction, so much so that even the fastest traveling particle in the universe (photons) are not fast enough to escape the attraction of the black hole.
Blackholes could not be fully understood until Einstein proposed the theory of Relativity. Even though Einstein himself did not believe that there could be points of gravity that could trap light, his theory allowed for the concept of space-time to come into play. If gravity is thought of as a simple "warping" of the fabric of space time, then a blackhole is simply a dimple that is infinitely deep. Since light is a particle as well as a wave it can be affected by this warping of space time as can any other thing with mass. Thus, with the proposal of relativity, Einstein allowed for the existence of blackholes in our universe.
If a black hole has enough gravitational attraction to pull in light, then how can we possibly find one? Everything that we see in the sky is emitting something, that's how we tell that its there. Fortunately, General Relativity solves this problem as well. In the theory when a particle is spun inward another exactly opposite particle is created, a "virtual" particle. When photons and other particles get captured in the event horizon of the black hole, the point at which the gravity becomes inescapable, a virtual particle is created just outside this event horizon travelling the opposite direction. This is one way that blackholes give off radiation. The other way is much more easily detectable. The matter spinning inward on the accretion disk of the blackhole is super heated by the gravitational forces acting on it from the blackhole. This heating produces x-rays that are given off outside the event horizon and thus can escape the grasp of the blackhole. With these two ways of "seeing" blackholes, blackholes all in all are not so black.
Blackholes will continue to facisnate theorists for some time. The gravitational effects of the super dense singularity (collapsed star) play havoc with space-time and produce interesting effects. Due to relativity anyone falling into a blackhole will have time slow compared to any outside observer. Of course it would tend not to matter since they would be "spagettified" by the gravitational tides anyway. However, beyond the event horizon of a blackhole, the effects space and time are truly unknown.