Together, the orcs and the tauren protected Cairne's village from the centaurs. When the fighting calmed down, Cairne told Thrall that he wanted to take his tribe to Mulgore. The leaders struck a deal -- Thrall's orcs would escort the tauren to Mulgore if Cairne would tell Thrall the exact location of the oracle. This was a pretty good deal for Cairne, who only had to tell Thrall what he already knew, while Thrall had to fight off a ton of centaur. And thus the cows befriended the ugly green aliens.
As promised, the orcs escorted the tauren caravans on their move to Mulgore. Despite some attacks, the taurens arrived safely and began to unpack their village. Cairne told Thrall that the oracle was in Stonetalon Peak, gave him the map coordinates, and told him to watch out for elites. The orcs headed north to Stonetalon while the tauren settled in to their new home. But Cairne decided that he felt bad about letting Thrall go off on his lonesome, since the tauren now owed a debt to Thrall for saving them, and went to Stonetalon himself. (Must have taken the route through Desolace.)
Thrall was pretty shocked to find Cairne waiting for him in Stonetalon, but the tauren leaders (and his friendly wyverns) were just what the orcs needed to take the peak back from the humans and get inside. Unbeknownst to Thrall and Cairne, the human Jaina Proudmoore had also been summoned to the oracle and was waiting inside. The little human mage was surprised when the giant orc and tauren stumbled into her in the caverns. They were about to fight when the oracle suddenly spoke to them. "Listen, you idiots," he said. "I'm Medivh, and I once almost succeeded in destroying Azeroth. Now the Burning Legion is coming, and you have to hang together or you're all going to hang separately. Incidentally, Thrall, doesn't Jaina look nice today? You guys should go get some coffee or something." The three agreed to work together to help defeat their enemies.
Cairne, Thrall and Jaina helped return Thrall's old pal Grom Hellscream to sanity, resulting in the death of the demon lord Mannoroth. Then the night elves Malfurion and Tyrande joined up with the ragtag alliance, again under orders from Medivh, who was probably getting tired of ordering enemies to work together. Then there's sort of a weird gap -- while Thrall, Jaina and the elves fought Archimonde on Mount Hyjal, Cairne just kind of disappeared. Then again, Cairne had just kind of ditched his people back in Mulgore, so maybe he thought it was time to go home and clean up while everyone else took care of the evil. There were some tauren at Hyjal, though. It's confusing.
Anyway, Archimonde fell at Hyjal, and Cairne went off to help Thrall build a new home for the orcs in Durotar. After Orgrimmar was completed, he headed back to Mulgore for some well-deserved rest and rebuilding. The tauren weren't able to build a nice place like Orgrimmar yet, but they were able to construct some villages to defend themselves from harpies and centaur. But then, one night, Cairne's son, Baine Bloodhoof, was captured by a band of centaurs. How does Cairne have a son? Is he married? Is anyone in Warcraft married, or do the kids just sort of spontaneously generate?
Cairne became really depressed after his son was kidnapped, and basically stopped leading the Bloodhoof. He just sat around, watching TV and scratching himself, a behavior that can still be seen today in the male tauren. At the same time as this, a force of humans led by Admiral Daelin Proudmoore was trying to drive the orcs out of Durotar. Thrall dispatched the heroes Rexxar and Rokhan to ask Cairne for assistance. When they arrived, Cairne told them he would not help, and asked them to tell Thrall that Cairne had died. Awww. Two other leading tauren, Tagar and Bovan Windtotem, told Rexxar and Rokhan about what had happened to Baine. Together, they managed to reclaim Baine from the centaur, snapping Cairne out of his depression. Cairne happily assisted Thrall and Rexxar's armies and defeated the humans.
Finally done fighting, Cairne went back to Mulgore. He decided to invite the rest of the nomadic tauren tribes to join the Bloodhoof in their grassy paradise, and except for a few rogue bands of Grimtotem tauren, the cow people united as one under Cairne's command. The tauren built their beautiful and easy-to-fall-off-of city of Thunder Bluff, where Cairne now rules as a benevolent chieftain.
Identifying features: Like all tauren, Cairne is about twelve feet tall. His fur is gray, and he wears a single plait as a beard and a nose ring. He also carries a big tree trunk as a totem. He's sort of a mixture of a warrior and a shaman at this point, because he can also wield an axe. Cairne is 104 years old now, and rumor has it that he'd be willing to give up the reigns to a worthy successor.
How you can kill him: Cairne is definitely killable, just like all faction bosses. I'm a Horde-side stalwart, so I've never done it, but according to WoWWiki, he's one of the easiest faction leaders. He thunderclaps and charges people. I've heard he also drop-kicks people off the bluffs, which is totally awesome if true.
For more information: Wikipedia on Cairne, WoWWiki on Cairne, Blizzplanet on tauren history.