Occlumency
about
*Taken from The Harry Potter Lexicon
Occlumency is the art of magically defending the mind against external penetration, sealing it against magical intrusion and influence - the defensive counter to Legilimency. A practitioner of Occlumency is referred to as an Occlumens.
Occlumency is a necessary prerequisite to defeat a Legilimens' lie-detector abilities without suspicious behaviour such as avoiding face-to-face contact and eye contact. Elementary Occlumency involves clearing the mind of thought and emotion, so that the Legilimens can find no emotional ties to memories that the target wishes to conceal. Simple resistance to attack requires similar skills to those needed to resist the Imperius Curse. In its more advanced form, Occlumency allows the user to suppress only feelings and memories that contradict what the user wishes a Legilimens to believe, thus allowing the Occlumens to lie without self-betrayal (OP24).
Occlumency has not often been studied at Hogwarts in the years prior to the second war against Voldemort, particularly not below N.E.W.T. level.
Occlumens
Severus Snape was a particularly skilled Occlumens, presumably more so than even Albus Dumbledore. His talent was especially necessary, as he spied on Lord Voldemort for the Order of the Phoenix. When Harry Potter began seeing into Voldemort's mind, Dumbledore arranged for Snape to give Harry lessons in Occlumency, so that Voldemort would not be able to use this connection to his advantage. However, Harry lacked the will to master Occlumency and rarely practiced, even though he proved to be very good at throwing off the Imperious Curse (a talent to which Snape said Occlumency was slightly similar). He did manage to block Voldemort's thoughts two years later, when he felt strong emotion (such as grief).
Bellatrix Lestrange, apparently, was also skilled in Occlumency. When Snape tried to see into Draco Malfoy's mind (while Malfoy was plotting Dumbledore's death), Malfoy was able to block him from his thoughts. Snape realized that Bellatrix (Malfoy's aunt who distrusted Snape) had been teaching him how to apply Occlumency.