Born Edward Alexander Crowley, he is primarily known for his occult writings and teachings. He founded the religion of Thelema, which became adopted by the Ordo Templis Orientis (O.T.O.) as well as the magical order Argenteum Astrum, the Order of the Silver Star. He was also a highly controversial member of the Hermetic Order of the Golden Dawn, where he was known by the magical name of Frater Perdurabo.
Crowley's lifestyle was absolutely shocking in the era in which he lived. Besides his interest in the occult, he was sexually promiscuous with both genders (at a time when homosexuality was still illegal in Britain), frequented prostitutes, was vocally defiant against Christianity and Victorian and post-Victorian prudishness toward sexual subjects, and was a drug addict.
While Crowley detested Christianity, he considered himself an immensely religious and spiritual person. His writings record incidents of experiencing deity, and Thelemites consider him to be a prophet. In 1904, he encountered a being known as Aiwass, described as a "minister" to Horus, the central deity in Thelema, and as a Holy Guardian Angel. Aiwass dictated the Book of the Law, which Crowley wrote down and published, becoming the central Thelemic text.
Crowley's beliefs included pursuing the Great Work, which included gaining self-knowledge and uniting with the larger universe. He also encouraged seeking out one's ultimate destiny or purpose, commonly referred to as one's True Will. (Excerpt from altreligion.about.com.)