Occultism could be described as the study or practice of the occult (meaning 'hidden') arts, which include alchemy, astrology, divination and all types of magic / witchcraft. At its root is a belief that the Universe is ruled by hidden forces and hidden laws, and that these forces and laws can be studied and applied, in the same way the other forces of nature, like, e.g., electricity, can be used.
The word is more or less a synonym of esotericism.