Black holes are a perennial favorite among physics buffs, who by now have the usual facts about such objects down pat: Most black holes form when stars explode as supernovae; all that matter collapsing into a dense object from which nothing, not even light, can escape, because of the strong gravitational effects. Lurking deep within a black hole is a point that is infinitely small and dense called the singularity.
While nothing can escape a black hole once it crosses the event horizon, thanks to a peculiar quirk of the quantum vacuum, it evaporates over time, emitting radiation (Hawking radiation) in the process - and how long it takes for the black hole to evaporate depends on its size (the bigger it is, the longer it takes to evaporate).
Oh, and if, say, an astronaut happened to accidentally cross the event horizon, he or she would technically be in freefall and thus wouldn't notice anything particularly unusual - not at first. It's only as said astronaut approached the singularity that gravity would become so extreme, s/he would be "spaghettified."