Occultism is a social movement based on the study of the occult, or hidden wisdom. It has seen countless resurgances throughout history due to the fact that people always seem to search everywhere for meaning in their lives. The most recent resurgance is in a large way due to movies like Necronomicon, and The Craft, and works from authors like Aleister Crowley, Robert Anton Wilson, and Israel Regardie.
It is a 'grey' area, perhaps larger than any other in the realm of religion. It can deal with subjects ranging from talismans, magic, and voudou, to ESP, numerology, and lucid dreams. It is all encompassing in that most everything that isn't claimed by any of the major religions is included in the realm of the occult. Even qabalah has been considered an occult study, perhaps of its popularity amongst magi and Wiccans.
See also New Age