A cult is a religious group (though not all cults are religious1) that follows a particular theological system. From the view of Christianity, a non-christian cult distorts the doctrines that affect salvation sufficiently to cause it to be unattainable. For example, it is an essential doctrine of Christianity that there is only one God (Isaiah 43:10; 44:6; 44:8; 45:5) and that believing in a false God brings judgment (Exodus 20:1-6). If a group were to affirm that there is more than one God (i.e., Mormonism), then it would violate an essential doctrine and be outside the Christian faith. Another essential would be that Jesus is God in flesh (John 1:1,14; Col. 2:9) and that to deny it means a person will die in his sins, John 8:24. The Jehovah's Witnesses deny Christ's deity and are, therefore, not Christian.
Apostasy (falling away from the truth) is different than being a non-Christian cult. Roman Catholicism, for example, is not a cult but is apostate. It has fallen away from the true Christian faith by violating the teaching of salvation by grace alone, through faith alone, in Christ alone which is something that all cults also do.
1. UFO cults are those groups that are dedicated to the advancement, study, and promotion of aliens, perceived alien communications, and or beliefs therein.