It is a playing card, the third most powerful one in a deck. It is a metal object that one picks up in a game of jacks. And it is the fictional hero in the LOST series.
In the series, Jack suits a card analogy because he had a Heart with Kate; he was a Club trying to get everyone to work together as one; he was a Diamond because he was a valuable person due to his medical skills, and a Spade, a worker who did not mind getting his hands dirty.
But throughout the various time lines of the Jack character, a different pattern emerges.
There were actually several different Jacks:
PRE-FLIGHT 815 JACK
ISLAND JACK
O6 JACK
TIME SKIP JACK
POST-SKIP JACK
DEAD JACK
Just to add a bit of confusion, they may not have all been the same person. We know that because Dead Jack was not "real" Jack but the alleged embodiment of his soul.
Pre-flight Jack was a dutiful son. He was a respected surgeon at an LA hospital. He was a miracle worker. He married Sarah, a patient, but soon jealously ruined his relationship. His marriage failed and then his professional reputation was tarnished trying to cover for his drunk father. Then, there may have been the odd Thailand/drug/runaway period of Tattoo Jack which may be the signal that his "perfect" life was going to get very, very dark.
Island Jack was still a troubled and upset person. He had trouble returning his father's body back to the States. He felt regret. But when the plane crashed and people were injured on the beach, his ER training took over and he began to lead the shocked passengers to safety. As a result, he was cast as the reluctant leader (something his father said he could never be because he could not "let go" and let patients die). On the island, he was challenged, manipulated, captured, beaten, right and wrong.
O6 Jack was not the same man before Flight 815. He put in place the great lie with an illogical construction to tell the world of their rescue which made no sense in protecting his island friends whom he had to believe were dead when the island disappeared. His life turned south very quickly; another failed relationship with Kate. He became addicted to drugs to the point of suicide. At his lowest point, he felt determined to return to a place, the island, for no apparent reason.
Upon his return, Time Skip Jack turned into a meek follower, a janitor who could care less about changing his situation. He was resigned to his fate at that point. He saw Sawyer change in the three years since the O6 left. Even though he had his old friends around him, he was distant and cold.
Upon the time skip to normal island time, Post-Skip Jack slugged along as a follower not knowing what to do. He was upset with the fact that he was an unwilling candidate in Jacob's game. He felt pressure to conform to someone else's rules. In order to get Jack out his funk, it appears that the danger, death and destruction elements were ramped up by the island (Jacob and MIB). Near the end, Jack relented an volunteered to be the next island guardian. He went into the cave to save Desmond and to reboot the island. As a result, he helped defeat MIB and allegedly save the world from something bad. But the cost, apparently, was his life.
Dead Jack probably had the best life of all the Jacks, which is quite ironic. Dead Jack had a good marriage with Juliet, which led to a son. Even after his divorce, Juliet and Jack were still good friends and sharing parents (even though Jack's son resented his father's long work hours and lack of interest in his life). Jack was again on top of his career path, head of spinal surgery. And just as Jack reconciles with his son, the Dead Jack story line abruptly ends at the Widmore concert when Kate shows up to bring him to the church where he finds out everyone is dead. It is actually a cruel trick that Dead Jack's idyllic sideways world life was totally fake. But Jack's reaction was equally confusing: he was merely numb by the fact that he was dead. He did not seem happy or content. He only briefly mingled with the island characters before sitting in the first pew, staring off into nothingness.
