According to the DC Comics Universe, the Lazarus Pit is a "mythical vat of arcane chemicals first discovered by Ra's al Ghul as a cure for the dying son of a sultan he worked for. The Pit had a terrible side effect, however; it turned the Prince temporarily insane, leading to him killing the doctor's wife. After being blamed for the murder, the man who would become Ra's Al Ghul was buried in the desert. He was rescued by a tribe and a boy named Huew. After this, he used the Lazarus Pit to save himself, and vowed to avenge his wife. The doctor gave himself the title Ra's al Ghul - The Demon's Head."
It is not known how The Lazarus Pit will play into the plot of The Dark Knight Rises.
The Dark Knight Rises was released July 20th, 2012 and stars Tom Hardy, Christian Bale, Anne Hathaway, Liam Neeson, Joseph Gordon-Levitt, Gary Oldman, Marion Cotillard, Morgan Freeman. The film is directed by Christopher Nolan.