Biblically, the occult is any practice that tries to gain supernatural power, abilities, or knowledge apart from the creator God. The Bible is clear regarding what the Eternal thinks about man's foolish attempts to circumvent Him.
When you come to the land which the LORD your God gives you, you shall not learn to do according to the abominations of those nations.
There shall not be found among you anyone who makes his son or his daughter to pass through the fire (child sacrifices), or that uses divination (an occult practice), or an observer of times (one who foretells lucky or unlucky days), or a fortune teller, or a witch (sorcerer)
Or a charmer (one who uses spells), or a consulter with familiar spirits, or a wizard, or one who seeks oracles from the dead (the occult practice of contacting those who have died). For all that do these things are an abomination (morally disgusting and abhorrent) to the LORD (Deuteronomy 18:9 - 12, HBFV)
A person can be involved in activities others find occult-like without actually belonging to a cult-like group. On the other hand, a person can be a member of a cult yet not involve himself or herself in the occult.
There are a variety of beliefs and practices that are part of the occult. Some of the more popular manifestations include astrology, white (sometimes called 'good') and black (sometimes labeled 'bad' or 'evil') magic, clairvoyance, dream interpretation, fortune telling, horoscopes, references to 'karma,' mediums, the study of metaphysics and necromancy.
Other occult activities and beliefs include Ouija boards, psychic healings, reincarnation, voodoo, seances, sorcery, spells, Tarot cards, witchcraft (e.g. Wicca), reading palms and Spiritualism (the belief that dead spirits can be contacted, usually through a medium).
The belief that spells, magic, strange gestures, etc. can alter or manipulate the physical world, including the minds of people, and bring prosperity and power, is also of the occult.