Along with Gopher's info, there is this from S.I. Hayakawa:
Sect may designate a smaller group within a denomination, especially one that differs from the larger body in a particular matter of faith or worship. Sect is also used derogatorily of a relatively small, unordthodox denomination, to stress its separateness or peculiarity: odd sects that spring up and bank on emotional appeal. The word sect acquired this derogatory connotation as a result of the many historical instances where sects were formed by groups that had split from their parent religion because of doctrinal discontent: the sects of the Gnostics.
The term cult is often applied to the forms or followers of a religious system that is looked on with suspicion or disfavor: The cult of Dionysius inspired orgiastic revelry during the celebrations of the Eleusinian mysteries. It also denotes a kind of worship or veneration that is not theistic in principle and is sometimes faddish in practice: the cult of nature; teenage worship that found momentary expression in the cult of James Dean.
The NT has a number of Greek words that apply here, primarily those translated schism and heresy. A sect is rather more like a schism: distinguished apart in a given religion or church. A heresy, like a cult, distinguishes itself apart from a given religion or church.
The WTS vehemently denies being a derivative of any other church (Adventist though they are in root and stock), so properly speaking they are not a sect, (or, a denomination: a rather more neutral term, but one that relates the given church to a common core of doctrine with other churches, as in "Protestant" denomination).
After getting past the "religion is a snare and a racket" campaign under Rutherford, the WTS is apparently quite happy to be called a "religion," but only if designated as "true religion." However, if only because they are a religion "looked on with suspicion or disfavor," they are a cult.
As far as James 1:27, it is perhaps worth noting that the Greek word used there for "religion, form of worship" is the most superficial of all the words James could have chosen. There are several other NT words that embody the sense of "reverential awe, piety, godly fear." The word James uses only "signifies religion in its external aspect...especially the ceremonial service of religion" (Trench)
That's one thing the WTS has down to a fine art: the "form" of worship, without the spirit. If for no other reason, they have earned the right and deserve to be called a "cult."