Sect is a general definition for "anything fragmented off a pre-existing religion". Cults can be sects and sects can be cults but not all cults are sects and not all sects are cults.
Here are some broad categories that I just made up on the spot that seem to sift them down into meaningful
Cults
of Personality
congeal around a single person and tend to be sects of a pre-existing religion or a synthesis of many religions where convenient to the divine personage. Example: Ramtha's School of Enlightenment, which tempts cultists with a pretty in-depth summer camp experience and promises of ESP for devotion.
Counterexample: The three messianic Abrahamic religions could all theoretically fall into this type but don't generally; this is because the Personality is dead and there are goals other than the edification of the divine personage.
of Esoterica
tend to be new religions that are advantageous to a small group or encourage specific behavior. General pagan tradition is cherrypicked to support a goal. Example: Dianic Wicca, which attempts to use faith as a bludgeon to drive in extreme Feminist beliefs by enshrining a great number of pro-female and feminine divinity viewpoints as doctrine.
Counterexample: A very similar religion, the Fellowship of Isis, does not subvert these cherrypicked viewpoints to a greater purpose. There is a great deal of overlap in membership but at not obvious point is there a rallying cry to unseat or otherwise disabuse the masculine, unlike the Dianites.
of Mystique
are relatively harmless until they're not. These are casual cults that people participate in often knowingly and willingly. It's not until they dig too deep in whatever weird lore they stole their schtick from and their beliefs start going sideways that things get real bad. Example: Heaven's Gate, a zany cali-cult with an emphasis on ayylmaos. You could argue it was a duo cult of personality but the actual draw in was that the worshipers were already big into UFOs and UFO lore and just wanted to make something more of it. Things got especially zany when the head honcho and thirty eight other people killed themselves through suffocation.
Counterexample: The entirety of /x/ or nearly the entirety of worshiping Buddhists, both of which have no idea what they're talking about.
of Desperation
abuse the weakness of an individual to draw them in for the gain of a larger group. Many other cults are like this, but few treat desperation as the end rather than the means. Example: Post-Hubbard Scientology. He built a massive network of filching based on the almost intrinsic depression and anxiety of modern successful individuals. Modern religions are sometimes in this category depending on who's preaching and where.
Counterexample: Physical trainers, who do still rip you off, but can actually produce real and tangible results without organ donations or abducting you.
Notice that of these options listed, only Heaven's Gate is sectarian (based on Christianity immediately), and only really by technicality, and the counterexamples simply lack the actively exploitative components of the examples but otherwise aren't all that different. Aside from the "mass suicide" stuff.
