Not having time for a long analysis, here are some brief thoughts:
Denomination, sect, cult all basically denote a group different from a larger group. But we have to look at the connotation of the words, also; they are not interchangeable.
The larger group in question here is Christian churches.
I would say those churches that are mainly in agreement with historic Christian teachings would be called denominations; e.g., Roman Catholic, Anglican, Methodist, Baptist, Presbyterian, etc.
I would say that those churches that are heterodox, significantly deviating from historic Christian beliefs in some areas, are sects; e.g., JW's, Christian Science, Mormons, etc.
I would say that "cult" carries with it the idea of both heterodoxy and obsessiveness to the point where members are cut off from all reality, independent behavior, and normal social relations; e.g., David Koresh and followers, Jim Jones and company, etc.
In my book, JW's are definitely a sect, but not quite a cult--semi-cult, maybe, but they still go out for beer and pizza. Or they did when I was there. I'm sure some will disagree--fire away!
Bill
"If we all loved one another as much as we say we love God, I reckon there wouldn't be as much meanness in the world as there is."--from the movie Resurrection (1979)