Sheol is the Hebrew abode of the dead, or of departed spirits. It is a subterranean region of darkness, from which return is impossible.
The Judeo-Christian term hell comes from the Hebrew word "Sheol", which technically means landfill. The confusion over what this word actually means stems from the fact that the Hebrews really didn't have a set word to describe their underworld, so when referring to it they used words they thought might describe it, thus the word Sheol.
It is interesting to note that Hebrew landfills were very unsanitary and unpleasant when compared to modern landfills; these places were filled with rotting garbage and the Hebrews would periodically burn them down, however by that point they were generally so large that they would burn for weeks or even months. In other words they were fiery mountains of garbage.