Legilimency
"The mind is not a book, to be opened at will and examined at leisure. Thoughts are not etched on the inside of skulls, to be perused by any invader. The mind is a complex and many-layered thing... It is true, however, that those who have mastered Legilimency are able, under certain conditions, to delve into the minds of their victims and to interpret their findings correctly."-- Severus Snape in OotP
Legilimency, a branch of magic not normally taught at Hogwarts (at least, not at Ordinary Wizarding Level) is the ability to extract emotions and memories from another person's mind. Although the word literally translates as 'mind-reading', this is considered a naive interpretation of the art by its practitioners. This blend was created by joining forms of the Latin words legens (reading) and mens (mind) with -mancy (which means divination).
Someone who practises Legilimency is known as a Legilimens. A skilled Legilimens would be easily able to detect truth or deception and would also be able to tell what a person was thinking or remembering based on the concept of withdrawing images from the person's mind.
Known Legilimens
» Severus Snape: Is both a Legilimens and an Occlumens, and is apparently better at Occlumency than Voldemort is at Legilimency, which is how he has survived in his difficult role of double agent among the Death Eaters for so long.» Albus Dumbledore: Is also a Legilimens, describing himself as sufficiently skilled to know when he is being lied to. He is also sufficiently skilled in Occlumency to be able to teach it if he so wishes.
» Lord Voldemort/Tom Riddle: Is a highly skilled Legilimens, in the estimation of Severus Snape, to the point where Voldemort can nearly always tell when someone is lying to him. Unlike Snape, Voldemort is never shown speaking the incantation to use Legilimency, if his first appearance in Goblet of Fire and last scene in Order of the Phoenix are any indication. He is able to possess the bodies of snakes while retaining his own body, possibly through Legilimency. Through his connection with Harry, he was also able to use his ability to feed images and eventually false visions into Harry's dreams, when Harry's mind was most relaxed and vulnerable.
Credit for this information goes to Hp-Lexicon and Wikipedia.