Theologically, a cult is a religious group which claims harmony with Christianity but which either denies or misinterprets essential biblical doctrines;
…and/or, psychologically or behaviorally, a cult is a secular or religious group which tends to use extreme and unethical techniques of manipulation to recruit, assimilate, control and retain members.

Sociological definitions of the term cult
…include consideration of such factors as authoritarian leadership patterns, loyalty and commitment mechanisms, lifestyle characteristics, [and] conformity patterns (including the use of various sanctions in connection with those members who deviate).

Religious
Eastern Mystical: groups related to Hinduism, Buddhism and other pantheistic Eastern religions; examples in this category are Hare Krishnas and Self-Realization Fellowship.

Aberrant Christian: groups that claim to be Bible-based but which deviate in practice or belief, such as The Way International, the Boston Church of Christ and the Shepherding Movement.

Psychospiritual or Self-Improvement: groups offering seminars or workshops providing self- improvement or personal transformation (a growing cultic trend), includes Transcendental Meditation, Lifespring and The Forum (formerly est).

Eclectic/Syncretistic: a combination of several religious traditions, includes the Unification Church (”Moonies”) and the Church Universal and Triumphant.

Psychic/Occult/Astral: these groups offer ‘’secret wisdom” and ”lost truths;” examples include UFO cults and Edgar Cayce’s Association for Research and Enlightenment.

Established Cults: Bible-based, cultic religious movements which have achieved mainstream status; this would include Mormonism, Jehovah’s Witnesses and Christian Science.

Extremist/Political/Social Movements: groups cultic in the psychological or social sense which include the Aryan Nation, White Aryan Resistance and the Ku Klux Klan.

http://www.apologeticsindex.org/c09a04.html

There are countries that have a state religion codified into their laws, it is illegal to practise other religions.
However those countries might recognize sects or cults within the legally sanctioned religion.

It’s useful when you use the word cult or sect to define why it is seen as such.