Zul'jin was a young male troll of the Amani tribe, the main leaders of the Amani Empire. There's no information on whether or not he fought in the Troll Wars (as the conflict between the Amani and the elves and humans was called by the victors), but it's highly unlikely, since it happened two thousand years before the First War and I'm not sure trolls are that long-lived. What is known is that when he became leader, after the Troll Wars, he led some daring raids on the outskirts of Quel'thalas and managed to unite all the forest trolls together into a rebuilt Amani Empire. The forest trolls were still pretty bitter about their loss in the Troll Wars, and harbored a deep grudge towards the humans and the high elves. Or maybe they were just racist against anyone without green skin, I don't know.
Zul'jin was kind of a visionary leader, and made grandiose plans to unite ALL the troll tribes. But the invasion of the Horde into Azeroth put the kibosh on those plans. The jungle trolls were down there in the thick of the invasion, near STV, but the forest trolls were largely left alone in their disunited and battered state. The troll tribes stayed out of the conflict , watching from the sidelines with a keen eye.
After the First War "ended" with Gul'dan's submission, the destruction of Stormwind, and Orgrim Doomhammer's ascension to chief, the remains of the battered human kingdom of Stormwind moved up to Lordaeron to ask for help. All the humans and some of the dwarves united to form the Alliance of Lordaeron. They were quickly followed by the Old Horde, which began launching naval offensives and ground attacks that led them up to Silvermoon. At that point, Zul'jin and the forest trolls came out to meet the Horde and basically tell them to stop messing up the lawn.
Doomhammer thought the trolls' hatred of the humans could be a great asset, and formally invited them to join the Horde, which sounds like a contradiction in terms. Zul'jin refused at first, but when the elves of Quel'thalas joined the Alliance, he realized that the trolls were going to be between a rock and a hard place if the Alliance won, and would probably get wiped out in the resulting clean-up operation. This opinion was confirmed when the elves captured Zul'jin. He was rescued by orcish forces, and in gratitude, he and the forest trolls joined the Old Horde. The orcs also promised the trolls their sacred ground once Quel'thalas was destroyed, which seems like a nice bonus.
Zul'jin and the trolls fought fiercely for the Horde, attacking and burning the forests of Silvermoon. Hey, that must be why they're all reddish today! Well, probably not, but it's a nice thought. This caused the elves to throw their full strength into the war effort, driving the Horde back to the Hillsbrad Foothills with much ganking and corpse-camping, and eventually to the shores of the Wetlands.
As the Alliance retook Khaz Modan and headed towards Blackrock Spire, Orgrim Doomhammer prepared a daring last stand. But it would have to be done without the trolls, because as soon as it became clear the Horde wasn't going to come out on top, the trolls did a runner back to the north. Did they think they were just going to hide until it was all over? Zul'jin was very upset by this turn of events, which meant he could never get the ancestral lands back, and he disappeared. Rumors spread that he had been killed by Alleria Windrunner or had become a mercenary. He became a hero and a symbol of hope and resistance to all the troll tribes, which is why the jungle trolls of the current Horde still say "For Zul'jin!"
Years passed, and the Horde reformed under the leadership of Thrall. The forest trolls were too savage and barbaric to join up with their Darkspear brothers in the new, clean Horde. One small, weak tribe of forest trolls, the Revantusks, did take up Thrall's offer to become part of the Horde, and they provided some clues to Zul'jin's location. He has gone missing, and all the forest trolls wait patiently for his return.
Where he is now: Zul'jin's failure in the Second War seems to have given him a renewed desire to destroy the high elves. He still leads the Amani tribe from inside the forest fortress of Zul'Aman, where he is aided by four animal loa spirits and the Hexlord Jin'zakk. It seems that he will be a killable enemy.
Wait, what? I'm a troll member of the Horde, and we revere Zul'jin! Why would we want to kill him?: Sorry, trolls, your reverence for your leader has been eclipsed by political necessity. The trolls of Zul'Aman have been attacking the blood elves of Quel'thalas, and as new members of the Horde, they're entitled to call upon Thrall for protection. Zul'jin is a member of the Old Horde, not the New Horde, so Thrall and company owe him nothing. Your worship of Zul'jin is misguided, anyway, since the forest trolls hate jungle trolls. In conclusion, find a new hero.
Fine, fine. While we're here, what's the deal with troll names?: Straight from the Warcraft RPG book: Zul is a troll prefix meaning "voodoo" or "voodoo master", and "jin" means tribal chief. Zul'Aman, Zul'Farrak, Zul'Gurub and Zul'jin are all thus named because of their roots in Voodoo, and Zul'jin, Sen'jin, and Vol'jin rhyme because they're tribal chiefs.
For more information: WoWWiki on Zul'jin, WoWWiki on Zul'Aman, Barb's screenshots from Zul'Aman.