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.
Between World War I and World War II the centre of occult and mystical activity was shifted from France to Belgium. Belgium became the main centre for many brotherhoods and secret societies of which many branches still exist today.
See also New Age, Theosophy, Anthroposophy, List of occultists, Esotericism, Gnosticism, Hermeticism