The Allegory of the Cave - from Platos Republic and c/o Wikipedia
This drawing is highly simplified and should only be used as an aid for grasping the picture the allegory creates; it does not represent the entire allegory.
Imagine prisoners who have been chained since their childhood deep inside a cave: not only are their arms and legs unmovable because of chains; their heads are chained in one direction as well so that their gaze is fixed on a wall.
Behind the prisoners is an enormous fire, and between the fire and the prisoners is a raised walkway, along which puppets of various animals, plants, and other things are moved along. The puppets cast shadows on the wall, and the prisoners watch these shadows. When one of the puppet-carriers speaks, an echo against the wall causes the prisoners to believe that the words come from the shadows.
The prisoners engage in what appears to us to be a game: naming the shapes as they come by. This, however, is the only reality that they know, even though they are seeing merely shadows of objects. They are thus conditioned to judge the quality of one another by their skill in quickly naming the shapes and dislike those who play poorly.
Suppose a prisoner is released and compelled to stand up and turn around. At that moment his eyes will be blinded by the sunlight coming into the cave from its entrance, and the shapes passing by will appear less real than their shadows.
The last object he would be able to see is the sun, which, in time, he would learn to see as the object that provides the seasons and the courses of the year, presides over all things in the visible region, and is in some way the cause of all these things that he has seen.
(This part of the allegory, incidentally, closely relates to Plato's metaphor of the sun which occurs near the end of The Republic, Book VI.) 
Once enlightened, so to speak, the freed prisoner would not want to return to the cave to free "his fellow bondsmen," but would be compelled to do so. Another problem lies in the other prisoners not wanting to be freed: descending back into the cave would require that the freed prisoner's eyes adjust again, and for a time, he would be one of the ones identifying shapes on the wall. His eyes would be swamped by the darkness, and would take time to become acclimated. Therefore, he would not be able to identify the shapes on the wall as well as the other prisoners, making it seem as if his being taken to the surface completely ruined his eyesight. (The Republic bk. VII, 516b-c; trans. Paul Shorey).