The quick way of describing tritons is that they are merfolk with two tails/legs (or leg-tails) originally from the Plane of Water—a touch more powerful/magical maybe (they can summon dolphins or Small water elementals), but that’s about it.
A closer look (borne out in Pathfinder’s Bestiary 2 and older D&D books like Jim Bambra’s PC3 The Sea People) is that tritons are the coral-builders and nation-makers. Merfolk tend to be more nomadic or even dwell in fresh water, while tritons are creatures of the great coastal shelves, building coral cities and guarding against evil underwater threats. (This tendency is accentuated in Pathfinder, where merfolk are more neutral to the point of hostility compared to the world’s oldest role-playing game. Similarly whereas on Mystara and Toril aquatic elves were forces to be reckoned with, on Golarion they are a diminished people barely holding their own against sahuagin and other threats.)
But for me, the two things that set tritons apart are these:
1) They’re the ones who are going to rescue you. Aquatic adventures can go bad fast. You’ve already used up a bunch of your resources buying potions or casting spells of water breathing (a 3rd-level spell, so not cheap). If something goes wrong, you’re stuck far from help and risk drowning or being crushed by the pressure. Tritons are your GM’s excuse to pull you out of the water. Merfolk don’t care and aquatic elves are too scattered. Only tritons have the organized patrols, territory, and the goodwill to plausibly be in the vicinity when you need a rescue.
2) Tritons are keeping the surface world safe. From aboleths to adaros to krakens to sahuagin to shoggoths, the list of evil underwater threats is long. If, like Earth, 70% of your game world is covered in water, it’s tritons that are making the beaches safe for swimming. PCs thinking that they are the ones saving the world all the time should get a rude awakening when they go underwater and encounter their first triton patrols.
In fact, as a GM you can really mess with them. Ever see one of those movies where the real stakes aren’t clear till halfway through second reel? (I’m trying to think of a better example, but for right now let’s go with that moment in From Dusk Till Dawn when a criminal misfits road movie suddenly becomes a vampire killfest.) Imagine your players going charging in to defeat a kraken after weeks of preparation…only to discover a triton army encampment about to embark on a battle against the real threat…and if the PCs want to be in their waters, they better come along.
Of course, that’s when tritons are on your side. Just because they’re neutral good, that doesn’t mean you can’t piss them off…
“You want to claim the wreck of the Hester? You have to earn it.” So says the triton captain who intercepts adventurers trying to plumb Daemon Gorge. With sea cats, giant moray eels, and cannibalistic merfolk about—not to mention the rumored daemons that give the gorge its name—the captain is not about to let them pass. Unless they can do a certain courier job for him…
When devils attacked the triton city of Davos, they triggered an epic contingency effect that sealed the city in a protective sphere…but the magical backlash flushed the city into the oceans of the Abyss. Now the devils and tritons (and their fiendish triton offspring) must work together to power the magical dynamos that camouflage Davos from the Abyss itself. The peace has held for years, but as more and more tritons are born tainted with Infernal and Abyssal energies, conflict is inevitable. And into this mix comes a band of surface-dwelling adventurers on a plane-hopping barque…
Eight feet high, barnacle-chested, and blessed by the Daughters of the Current, Stefanic Landstrider is known above and below the waves as a triton champion. But when he begins acting madly—rumors whisper of lycanthropy, possession, aboleth enthrallment, or worse—someone needs to bring him in…or put him down. No triton will lay a finned finger on him, so their duke recruits a band of adventurers who have helped him in the past.
—Pathfinder Bestiary 2 270
Tritons have popped up here and there in the Golarion books. A pretty much full list is here; your first stop should be “Oceans of Golarion” in Pathfinder Adventure Path #56: Raiders of the Fever Sea.