Now that we've got that out of the way, the story can begin. Many, many years ago, there was a troll tribe called the Zandalar. These were the original trolls, and they valued knowledge, spirituality, and peace. They built a beautiful capital city named Zuldazar on the content of Kalimdor, which was at that time the only continent on Azeroth. Some more warlike trolls decided to split off from the priest-based society of the Zandalar and headed to the far north and far south of Kalimdor. There they founded two warring empires: the Amani Empire and the Gurubashi Empire, named after the troll tribes at their heads, but better known as the forest and jungle trolls.
The troll tribes became bitter enemies, but faced a greater challenge when a new race came along: the magical night elves. (There was also something in there about fighting the Qiraji insectoids, but like everything else with the Qiraji, it sucks and we're going to pretend it never happened.) The night elves quickly dismantled the troll empires while expanding their own territories. Defeated, the trolls developed a seething hatred for the elves, which was not improved when the Burning Legion came to town and the Well of Eternity imploded, sundering the continent. The Zandalar tribe was all right, since their city had somehow survived intact to become an island and a troll mecca in the South Seas. However, the jungle trolls and the forest trolls didn't have it quite as good.
Jungle trolls: The two troll empires managed to rebuild some of their old territories in the Ghostlands and Stranglethorn Vale, but the forests were tough and they barely survived. The Gurubashi Empire turned to worshipping the god Hakkar, who helped them expand their territory across much of the continent. But the god turned into a bitter and cranky deity, demanding blood sacrifices and finally asking to be summoned into the world. The Atal'ai and Hakkari priests acquiesed for different reasons, and built the Temple of Atal'Hakkar and (eventually) Zul'Gurub. The rest of the jungle trolls disavowed and attacked these priests while continuing to fight among themselves.
Eventually, the small Darkspear tribe got sick of getting picked on and set sail across the sea, where they landed in a cove off of Kalimdor. One day, a fleet of green-skinned, stocky creatures showed up on the island after their ships were damaged in a storm. The greenskins were orcs, and they and their leader, Thrall, were welcomed by the Darkspears and their chief, Sen'jin. Sen'jin warned Thrall that the island held many dangers, including ... MRGLEMRGLEMRGLE. A tribe of murlocs captured Thrall, Sen'jin, and some of their countrymen. The murlocs imprisoned everyone but Sen'jin, whom they took away to sacrifice to a sea witch. Thrall and his men broke out of the prison, but they were too late to save Sen'jin, who was dying from his sacrificial wounds. Sen'jin told Thrall he had seen a vision of Thrall leading his people to Kalimdor.
To repay the trolls for helping his people, Thrall offered them a place in the New Horde, and a home in Durotar. Vol'jin, the son of Sen'jin and the new troll leader, accepted the offered and set sail for Kalimdor. They created a new homeland in the Echo Isles. However, a troll witch doctor, Zalazane, started turning the Darkspear trolls into zombies. Fearing for his people, the (level 70 elite) Vol'jin ordered his (levels 12-14) people to abandon their new land to the (level 10 non-elite) Zalazane. The trolls created Sen'jin Village instead, which still stands today as a monument to Vol'jin's inability to do simple math.
Forest Trolls: Unlike the jungle trolls, the forest trollsdidn't fall into worshipping evil gods, preferring to stick with the simple, old-time religious practices of voodoo and cannibalism. Instead, their threat came from the outside. The Amani Empire had spread across most of northern Lordaeron, and the trolls were very happy until ... guess who showed up? Those damn arcane magic-addicted elves again, except this time they were called high elves. The trolls were still pretty upset about the world being destroyed and attacked the elves. The situation did not improve when the elves decided to make the city of Quel'thalas on top of some troll tribes, and also build magical runestones to drive the trolls further away.
The races lived in an uneasy stalemate for several thousand years. But during the reign of King Anasterian Sunstrider, the forest trolls decided to strike back and raid Quel'thalas. Outnumbered, Sunstrider called on the humans of Arathor to help him, and the new alliance shattered the troll empire. The trolls were understandably pretty angry at this, so when they found out that the humans and the elves had joined this newfangled "Alliance" that was fighting the "Horde" -- and the chieftain of the "Horde" offered to take them under his wing -- they joined together again and signed up for Hordedom under Zul'jin. They provided a lot of forces to the Horde, but when the Alliance turned the tables and defeated the Horde, the forest trolls abandoned their new allies and went home. (More on this next week.) A small group of forest trolls, the Revantusk Tribe, decided to join Thrall's new Horde, but the rest reverted to their savage ways.
Other trolls: Blizzard's troll history site has a fascinating section on all sorts of weird trolls. The ice trolls, found in Northrend and Dun Morogh, were exiled from the Zandalar for being brutal and bloodthirsty. They're the sworn enemies of the dwarves. The sand trolls ended up stranded in Tanaris after the Great Sundering, and got all sunburnt and red. They built the city of Zul'Farrak and worship a god named Gahr'zilla, which makes me feel bad about all the times I destroyed Zul'Farrak. I mean, we're pretty much doing the bidding of the expansionist goblin trade princes and destroying the native inhabitants of the land.
The dark trolls are rare trolls, sometimes thought to be a myth. They live underground, have a primitive society, and are seen as violent even for trolls. However, the dark trolls joined the night elves to fight the Burning Legion on the slopes of Mount Hyjal, showing that they cared about the fate of the world.
So what happened to the Zandalar? As mentioned earlier, the Zandalar were the first trolls and arguably the most civilized. While most of the trolls eventually became members of the Amani and Gurubashi empires, they still revered the Zandalar as the priests and history-keepers of the trolls. After the Great Sundering, the nation of Zandalar and its capital, Zuldazar, became an island in the South Seas due to the priests' protective spells, and the Zandalar retreated deep into their studies. Recently, a small group of Zandalar trolls has come to Yojamba Isle in Stranglethorn Vale. They're there to encourage adventurers to go into Zul'Gurub and defeat the murderous troll god, Hakkar the Soulflayer.
The most awesome thing about the Zandalar and Zuldazar is that their island home is a strictly neutral territory, where all trolls of different tribes can come and mingle without fear of attack. And they actually do that! Every six years, emissaries from every troll tribe come to a conference in Zuldazar to discuss troll issues, led by the troll King Rastakhan. I strongly suspect that if the South Seas/Undermine is ever added into WoW, Zuldazar will serve as (finally) a troll capital city.
I'm a member of the Alliance, and I think trolls are beastly, savage cannibals. Can you tell me anything that will make me hate myself? Okay. In all probability, night elves are descended from trolls. An in-game book, "The Twin Empires", contains the following passage: "Ancient texts speak of a small faction of trolls that broke off from the Amani Empire and founded their own colony in the heart of the dark continent. There, these brave pioneers discovered the cosmic Well of Eternity which transformed them into beings of immense power. Some legends suggest that these adventurous trolls were the first Night Elves, though this theory has never been proven."
Trolls are the first race on Azeroth, night elves are the second, and trolls and night elves have very similar appearances. It is strongly suspected that night elves are trolls that wandered too close to the Well of Eternity and mutated. However, night elves get very angry when you mention this, and say that it's a way for trolls to marginalize night elf accomplishments and deal with their defeat by the early elves. The night elves believe that they are descended from the goddess Elune herself, further proving my theory that elves are jerks.
For more information: Worldofwarcraft.com's Troll Compedium, a rainbow of trolls, speculative chart of elf/troll lineage.