There are two kinds of cults:
1. Psychological/sociological -- these seek to take over control of people's personal lives; they often manipulate people into a double bind situation where the only choice is to submit; they begin to make your decisions for you... money, family, jobs, education, entertainment, time, etc...
2. Theological -- these seek to redefine major biblical doctrines, usually associated with the godhead. Many deny the personality of the Holy Spirit. Others deny the eternal pre-existence and deity of Jesus. Some try to wrap the entire godhead into just one person.