After years of study, Jaina grew into a powerful sorceress under Antonidas's tutelage. One day, a prophet arrived at Dalaran and pleaded with Antonidas to send his people west to the continent of Kalimdor. Jaina thought the prophet might have a point, but Antonidas disagreed and sent her north to the village of Brill to investigate this newfangled plague thingy that was spreading around Lordaeron. Jaina and Arthas found that the plague of undeath was being spread through corrupted grain from the city of Andorhal and organized by the necromancer Kel'Thuzad. The duo chased Kel'Thuzad back to Andorhal, where Arthas "killed" him.
On the way back, Arthas and Jaina stopped to rest at the town of Hearthglen. Unfortunately, the citizens of Hearthglen had eaten the corrupted grain and were turning into the undead. Jaina rushed to get reinforcements from Uther Lightbringer's Order of the Silver Hand paladins while Arthas held off the zombies. The paladins got back to Hearthglen in time to save Arthas and win the day, but the prince had been deeply traumatized by what he saw in the city.
Arthas, Jaina and Uther went to the city of Stratholme to fight the dreadlord Mal'Ganis. But the same thing had happened there as in Hearthglen -- corrupted grain, citizens soon to become undead, etc. Arthas wanted to kill off the still-human Stratholme residents before they became undead, but Uther thought that went against every ideal of paladinhood. Arthas accused Uther of treason and demanded that anyone loyal to him and Lordaeron stay and kill the citizens. Jaina, horrified at Arthas's actions, followed Uther out, wondering what had happened to the caring young man she had courted. Uther and Jaina returned to the ruins of Stratholme after Arthas had burned it to the ground, where the mysterious prophet once again told her to go to Kalimdor. So in order to escape from the actions of her ex-boyfriend, Jaina packed up and headed to the opposite side of the world on a whim. Sounds like when I was twenty and went to Amsterdam, except with more magic and swords and less drinking and reading cheesy British chick lit.
When Jaina and her Kul Tiras forces arrived in Kalimdor, they were greeted by some hostile green folks. The orcs had decided that Kalimdor was going to be their new home, and they were not happy to see the people who had been imprisoning them for years. Jaina's forces fought with Grom Hellscream's orc troops, and the battle did not go well for the humans. Refusing to give up just yet, Jaina decided to go to the easily-defendable and strangely mystical Stonetalon Peak. She went inside the caverns of the peak to search for a power that would help her defeat the orcs. Instead, she ran into the leader of the orcs, along with the leader of the tauren. Jaina prepared to fight Thrall and Cairne. Let's see, a human mage against an orc shaman and a tauren ... shaman/warrior thing. Advantage: Horde, at least with the new PVP trinkets dispelling polymorph.
But before they could fight, the mysterious prophet who had been telling everyone to go to Kalimdor told the three leaders that they would all have to work together to fight the Burning Legion. They all reluctantly agreed and then trudged out of the mountain. Soon, Thrall's friend Grom Hellscream began acting strangely, and to cement their tentative alliance, Jaina used her magic to help Thrall free his comrade. At that time, the Burning Legion began invading Kalimdor, so the human and orc forces joined together out of necessity. After attacks by undead, demons, and those jerk night elves who kept hiding in the forest, the orcs and humans were about at the end of their rope. Then the prophet appeared again in a vision to Thrall, and led Thrall and Jaina where the night elf leaders Tyrande Whisperwind and Malfurion Stormrage were stationed. Finally, the prophet revealed himself as the spirit of Medivh, and told the leaders they had to join as a group to defeat the Legion. Then Thrall smacked him for delivering cryptic messages for months instead of just telling everyone what was going on. Wouldn't that have been great?
Anyway, Thrall, Jaina and the elves developed a trap for the Legion's commander on Azeroth, Archimonde. They knew he was going to the World Tree, the fount of magic on Azeroth. So they set up three bases -- human, orc, and elf -- on the mountain to delay him. Archimonde devastated all three bases, although Jaina used her magic to save herself and Thrall from death. Then when Archimonde got to the World Tree, Malfurion essentially blew it up. The orcs went to found their new land, the night elves went back to sleep, and Jaina took her refugees to a small island south of Durotar, where she founded the city of Theramore.
Theramore and Orgrimmar lived in an uneasy peace until groups of humans started raiding orc outposts on Durotar. When Rexxar asked Jaina what she knew about this, she denied any involvement and agreed to help the Horde investigate. After doing some scouting, they returned to Theramore, where they discovered that Admiral Daelin Proudmoore had decided to pay his daughter a visit. Proudmoore demanded that Rexxar and the other Horde members be arrested, but Jaina helped them escape. Proudmoore was enraged at his daughter's betrayal and took over the military forces of Theramore, using them to attack the orcs directly. Eventually, the Horde got their act together and fought their way to Theramore, where Jaina ordered the troops loyal to her not to attack. Devastated, Jaina stood by as Rexxar killed her father, knowing that the peace with the Horde was more important than her family bonds.
Where she is now: Jaina is the founder and widely considered ruler of Theramore. Theoretically, she's also the closest thing the Alliance has to a cross-racial faction leader -- she leads the humans of Lordaeron and is friends with the night elves. However, her friendship with Thrall means she'll probably never be acknowledged as a ruler. Recently in the "Cycle of Hatred" novel, Thrall and Jaina worked together with the guardian Aegwynn to defeat a minor demon, and Aegwynn became an advisor to Jaina.
Who she's seeing: Jaina is married to her magical studies -- not surprising considering her former boyfriends, one of whom is now working for the Burning Legion and somehow surviving being farmed for loot (yep, word is that Kael'thas somehow lives,) and the other of whom is the immortal Lich King and enemy of all life on Azeroth. Still, there are persistent fan rumors suggesting she will be linked romantically to Thrall, which is ... hmm. Those would not be pretty babies.
Identifying features: Jaina is an adult female human with shoulder-length blonde hair. She does not like physical fights, preferring to settle matters with her magic. As such, she enjoys teleporting Horde attackers out of her tower in Theramore and laughing when they splat on the rocks below.
For more information: WoWWiki on Jaina, Wikipedia on Jaina, Warcry's summary of Cycle of Hatred.