It is a basic Wizard power, not really a spell.
http://en.wikipedia.org/wiki/Occlumency#Legilimency_and_Occlumency
[edit] Legilimency and Occlumency
Legilimency is the magical skill of extracting feelings and memories from another person's mind — a form of magical "telepathy" (although Snape, an able practitioner of the art, dismisses the colloquial term as a drastic oversimplification). It also allows one to convey visions or memories to another person, whether real or imaginary. A witch or wizard possessing this skill is called a Legilimens, and can, for example, detect lies and deceit in another person, witness memories in another person's past, or "plant" false visions in another's mind.
The counter-skill to Legilimency is Occlumency (and its user, known as an Occlumens), by which one can compartmentalise one's emotions, or prevent a Legilimens from discovering thoughts or memories which contradict one's spoken words or actions.
Voldemort, Snape, and Dumbledore are all known to be quite skilled in Legilimency and Occlumency. Throughout the books, Snape is repeatedly said to be highly skilled in Occlumency. Voldemort is said to be the master of Legilimency by Snape, as he, in almost all cases, immediately knows during conversations if someone lies to him.
The skills are first mentioned in Order of the Phoenix, (though Harry gets the impression before that Snape can read minds) wherein Snape is instructed by Dumbledore to give Harry lessons in Occlumency. Whether as a result of negligence in Snape's instruction, or poor aptitude on Harry's part, Harry never made any progress in the skill, and as a result he was lured by Voldemort through a carefully calculated vision he falsely believed to be real. Only once has Harry managed to overcome Snape with the use of Occlumency, in Order of the Phoenix.[14] During the lesson, Harry notices that the Dementors become fainter and Snape becomes clearer.
Bellatrix Lestrange, Draco Malfoy, Narcissa Malfoy and Barty Crouch Jr may also have some skill in Occlumency, since Bellatrix was said to have taught Draco to shield his thoughts from Snape[HP6] and Narcissa's treachery is not detected when Voldemort asks her to check Harry's pulse after he has seemingly killed him.[HP7]
Legilimency and Occlumency are not part of the normal curriculum at Hogwarts, and most students would graduate without learning them. They seem to be considered a more advanced form of magic.