Some have members practice certain rituals or follow a set of principle rules. The group usually believes its way is the only correct way to live life, and all non-members are doomed to some horrible fate if they cannot be persuaded to join.
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.
Note: Not all cults are harmful in nature. Many are benign, i.e. trekkies.
See www.rickross.com for more extensive definitions.
2. A small or unconventional religion