Jack was a genetically-engineered Human. Jack was the unofficial leader of a group of genetically-engineered Humans who lived with him under psychiatric care by Dr. Karen Loews. Jack's parents took him, as a child, to undergo an illegal procedure called accelerated critical neural pathway formation which dramatically increased his intelligence beyond what Humans would call genius level. Jack also had superior physical abilities, such as enhanced agility. However, the procedure also turned Jack into a violent, anti-social, aggressive man who went on to have serious problems fitting in to mainstream Federation society.
Jack was taken in to psychiatric care and lived with others like himself. In 2374, after it was revealed that Doctor Julian Bashir had undergone the same procedure, Doctor Karen Loews - the doctor in charge of Jack's case - took Jack, along with Patrick, Lauren, and Sarina Douglas to Deep Space 9. Dr. Loews believed that meeting someone like them who had managed to fit in to mainstream society would be helpful to them.
Jack, however, objected, pointing out that Bashir only fit in because he hid his abilities and pretended to be "normal". Jack even threatened Sarina's life at one point.
Along with the others, Jack studied Gul Damar's speech calling for peace talks between the Dominion and the Federation. Together, just by watching the speech, they were able to determine the entire story of Damar's rise to power without ever having heard any of it before. Bashir was very impressed, and encouraged their thirst for knowledge about the Dominion. They asked to watch the holographic recordings of the peace talks, and determined that the Dominion was stalling for time to solve their shortages of ketracel-white. However, they also pointed out that, should the Federation refuse to sign the treaty, the Dominion would lose their only chance to secure a new supply of ketracel-white and, given the inevitable shortage, would have no choice but to send all the Jem-Hadar to their deaths in a devastating all-out suicide strike against the Federation.
Grateful to them, Starfleet provided Jack and the others with classified reports and war plans. They quickly determined that the Dominion was certain to win the war and that the only way to prevent the loss of billions of lives was to surrender immediately. Unsurprisingly, Captain Benjamin Sisko rejected this immediately. Jack, Patrick, and Lauren attempted to take Starfleet's classified information directly to the Dominion, knocking Bashir out and leaving him in Sarina's custody, so they could win the war more quickly, thus saving lives. Jack miscalculated, however, as Sarina released Bashir, who then stopped him from meeting with Weyoun. Starfleet, however, remained willing to listen to any ideas they had on how to win the war. (DS9: "Statistical Probabilities")
In 2375, Julian Bashir developed a procedure that would cure Sarina of her inability to properly communicate with others. With Patrick impersonating a Starfleet admiral, Jack and the others went to Deep Space 9, where their help proved invaluable for Bashir to perform the procedure successfully.
Jack also had fanciful ideas about preventing what he saw as the inevitable collapse of the universe in around sixty trillion years. He believed that by altering the cosmological constant via the use of a vast number of subspace generators placed throughout the universe, this collapse could be avoided. (DS9: "Chrysalis")