I've been beat to the punch! Others have pointed out several Jedi sects, but I'll offer my own take on what the Jedi get up to. As for the Sith, I'll defer to other folks!
First off, a few famous sects have already been mentioned; the Corellian Green Jedi probably being the most widely-known. They have some differences in philosophy, orthodoxy, and doctrine, though most of the core ontological views are pretty much the same.
So why is that? There are a few good reasons.
One is that the supernatural elements of Star Wars that these religions are based around (the Force) have been handed down pretty authoritatively. Debates and interpretation rage out of universe, but I don't think anyone can really deny that Yoda was laying things out pretty solidly in
Empire Strikes Back. There's no debate that the Force is energy created by living beings which binds the galaxy together, despite the in-universe objections of the occasional cynic. That so much of these religions' basis is empirical, objective fact as laid down by George Lucas naturally limits those religions' ability to form wild new interpretations, heresies, and epistemological beliefs -- the only Cathar you're likely to find in Star Wars are cat people, rather than
quasi-Christian gender-fluid apocalyptic cults.
Lacking much wiggle room for ontological shenanigans, instead, much of the natural point of disagreement on the Force lies in moral philosophy itself. This is what gives us the great schism of Jedi and Sith -- not a disagreement over ontology (which is largely irrelevant to the story of Star Wars), but over morality (which is the point of Star Wars!). Sects like the Green Jedi differ slightly in a few small details of moral interpretation, such as the permissibility of maintaining a family, which leads to another reason why heresy and sectarianism are less common in Star Wars.
The nature of the Force lends itself to creating a universe with a harsh, stark dichotomy to it (as discussed at great length
here -- long, but essential to understanding the moral consequences of a universe with the Force, IMO). It is an exaggeration and polarisation of the real world, 'bleaching' out the fuzzy, often incoherent moral actions of real people and reducing them to their fundamentals: do you choose to do
good or do you choose to do
evil? Nothing actually forces us to make this choice in real life, resulting in reality's messy line between good and evil, but in Star Wars the Force enforces (hyuck hyuck) this duality. If you use the incredible power of the Force to do
bad things, i.e., allow yourself to be corrupted by the power you wield by misusing it, that manifests in very physical and very tangible ways.
While I'm going to explain the problem with using Western religious analogies in a moment, I'm going to use one here for the sake of ease of understanding. You can liken the Good/Evil dichotomy imposed by the Force to the sort of apocalyptic visions in many Western religions' mythology, especially Christianity. If you choose Good, you sprout silver wings and become an Angel. If you choose Evil, you sprout horns and cloven hooves and become a Demon. In our hypothetical world of Angels vs Demons, you could probably spend uncounted hours pouring furious energy into worldbuilding a complex Angelic society with many different Angels and the different ways our Angels interpret Good... but at the end of the day, they're still all bewinged Angels. Likewise for our Demons.
In that context, I think you'd expect genuine heresy and sectarian differentiation to be uncommon relative to the real world. Not necessarily unheard of -- just relatively uncommon. An Angel is an Angel is an Angel, and a Demon is a Demon is a Demon*. You might have fallen Angels or rebellious Angels or unorthodox Angels, but they're all still Angels in at least some senses of the word. Thus sectarian difference becomes less distinctive, and therefore less relevant than the overarching duality of Angels vs Demons.
Apply this to Jedi and Sith, and you can see why something like the Corellian Green Jedi is both uncommon and doesn't really diverge from the main Jedi orthodoxy all that much.
There's a third problem with sectarianism that's more one of out-of-universe expectations. Namely, the natural assumptions of someone who grew up in Western civilisation. If you grew up in Western civilisation, it's a safe bet you're pretty familiar with Christian theology, worldview, and philosophy, whether you're Christian or not. Christian philosophy inundates Western civilisation and has helped shape its thinking for 1500 years. And while some elements of this thinking are certainly present in Star Wars to large degrees, the actual in-universe religion of Star Wars is more akin to traditional Eastern thinking than Western. The Force and the Jedi's inspirations are pretty well understood to be rooted heavily in Zen Buddhism and Taoism -- hence the focus on spiritual mysticism rather than gods and heaven and hell. If you do
bad things in Star Wars, you don't go to some afterlife Hell to burn for eternity... rather, you soil your own spirit. It's much more introspective, and not focused on external motivators like a God or a Devil.
So, comparing the Jedi or Sith to Western religious sects is inherently flawed. Instead, you have to look at the more spiritual religions like Buddhism if you want to make valid real-world comparisons. Now, Buddhism et al do have divisions within them, certainly, but remember that for the Jedi and the Sith, there are few, if any, questions about ontology or epistemology, which naturally cut down on the sort of sectarian divisions that might otherwise arise if you try to compare directly to Buddhism.
tl,dr; We've been handed the keys to a universe which very authoritatively stated the empiricism of its spiritual elements. Much like a fantasy world with obvious gods with obvious divine power, where if you stray from your temple's teachings your god literally stands before you, slaps you, and says 'Knock it off,' there is simply much less room for radical interpretation of your religion. There's not
no room, but there's
less room, so cults and sects are much less common. That wiggle room is almost entirely a moral dimension, framing the essential moral conflict of the Star Wars galaxy, which in turn is given something of a Zen Buddhist flavour rather than the Dante's Inferno most of us are inherently more familiar with.
*This is why I generally refer to Zakuul, its Knights, Valkorian and his family, as well as Snoke and Kylo Ren as
Sith, even if they're never explicitly referred to as such or specifically indulge in traditional Sith philosophy. If it walks like a duck...
