The term "cult" has several meanings, in a fairly basic and neutral sense it can indicate rite, worship, or honor. For example the cult of relics in Roman Catholicism speaks of the veneration of relics, the cult of saints the veneration of the saints, etc. In the pagan context of ancient Rome we can speak of the cult of Adonis, or the cult of Dionysus, or the cult of Jupiter.
In this sense I don't think calling Christianity would at all be wrong, as the cult centered upon the person of Jesus. Here "cult" is not a negative, but a neutral idea of worship, veneration. We still use this sense often when we talk about a "cult of personality" in regard to celebrities or public figures, in and of itself it is a neutral sense.
There is also a negative sense, used in the study of the sociology of religion, a cult is a religious organization or movement that has certain marks or qualities associated with it: cults are highly controlling, members are expected to devote themselves entirely to the group and especially to the group's leader, members' lives are dominated by the group and the leadership of the group is to be accepted without question, members are regularly expected to cut ties with the outside world and sever relationships with family members and friends who are not part of the group.
Jamestown (The People's Temple) was a cult. The Branch Davidians were a cult. The FLDS is a cult. The Church of Scientology is a cult.
In certain Christian polemical circles, namely the counter-cult movement, a "cult" gains a new definition largely as a group which does not conform to the historic orthodoxy of Christianity, and is therefore essentially an alternative term for a heretical or heterodox group. I don't know that this is a particularly helpful definition though, as often it is also conflated with cults as mentioned above, and also doesn't necessarily take into consideration cults which may adhere to a more orthodox theology.
The orthodoxy-heterodoxy dynamic isn't, in my opinion, a helpful way of defining a cult; further there is the risk of defining a cult entirely subjectively on the basis of "that group is different than my group". By restricting the idea of a cult to the sociological definition(s) I think we have something a bit more objective to work with; when Christians deal with the orthodoxy-heterodoxy dynamic it would be preferable to use those terms: heretical, heterodox, etc since the issue is a deviation or disagreement with an established theological orthodoxy.
Cults are [usually] religious groups which exercise control on an authoritarian level, and keep members in line through threats, emotional and psychological manipulation, separating members from the outside world so as to make them more dependent upon the group and more easily manipulated by the group's leadership. Whereas religion(s) tend to not exercise that level of control. If someone decided they didn't want to be a Lutheran, a Presbyterian, a Methodist, or a Catholic anymore their friends and family may be saddened by the choice, but the choice will most likely be respected and things will remain amicable--Lutherans or Methodists aren't going to send goons to a deconvert's house and threaten them or make life a living hell for them, and aren't going to prohibit them from spending time with family still in the churches; because mainstream churches like this don't exercise control like that, and aren't interested in manipulating people into staying lock-step, and typically leadership in these churches aren't "powerful" but act as public servants for the good of the congregation, and typically are subject to the congregation itself on some level.
-CryptoLutheran