Wikipedia has a nice article on cults. Including a collection of dictionary definitions. The definitions include a system of religious beliefs and ritual, A religion or sect considered to be false, unorthodox, or extremist, with members often living outside of conventional society under the direction of a charismatic leader leader, or a small religious group with excessive control over members.
So depending on which definition you pick either all religions are cults, all are cults expect a few major religions, or very few cults exist. In my personal opinion I consider cults to be the groups who seclude themselves from society, either in complexes, monasteries, or some other form of isolation. I only have problems with cults if they brainwash people into joining or have dangerous or destructive beliefs. From what I understand there are very few cults out there that actually brainwash or are destructive, but they do exist.
Nobody should be forced into believing anything. Sometimes when I talk to my religious friends I feel like they have been brain washed into believing in Christianity, and some of the more far fetched parts of the bible. I just can't seem to understand why people are so willing to believe that communion can be considered to literally be the body and blood of christ. My cousin was an alter boy. He showed me the giant package of "Communion Waffers" the ingredients list did not actually contain Jesus. When I listen to people who try to convince me they are eating Jesus, I can't help but get the feeling that all religions are brainwashing people.
Either way, I still consider a cult to the creepy little secluded groups of people, but major religions brainwash people too, just more openly.