A group, often times though not always religious or spiritual in nature, that is led by a single or small group of leader(s).
Members are often recruited or by some means persuaded to join, rarely if ever knowing how destructive and harmful a cult can be -- rarely knowing that it IS a cult. Though they usual come off as being generous, caring and in the best interests of their members, cults are inherently based on furthering the desires of cult leaders.
Cult leaders commonly use thought reform or "brainwashing", in conjunction with other methods, to slowly and deliberately reel in more control of said members. In many cases, members may eventually forsake their friends and family (non-members are viewed as "wrong" or "bad") and give up their careers, homes and/or money to the leader.
Jonestown is an infamous example of a cult. Many people were recruited to what was portrayed to be a friendly, all-inclusive and loving community church. Through years of exponential thought reform, however, members would eventually devote their lives to Jim Jones and, in effect, die for him.
Note: Not all cults are harmful in nature. Many are benign, i.e. trekkies
See www.rickross.com for more extensive definitions.