Jack was taken from her mother and abducted by cerberus at an early age due to having very strong biotic potential, her mother was later told that Jack had passed away due to complications from the very thing biotic powers stem from - elemental zero exposure.
Jack grew up under terrible conditions in captivity and was constantly subjected to cruel testing to test the boundaries of her biotic potential, while she was isolated for the most part there were other children being experimented on in the very same lab - however they were treated more as disposable lab rats while Jack (now referred to as Subject Zero) was the more valuable subject due to her much bigger potential.
Eventually Jack managed to escape and leave in a shuttle, killing almost everything and everyone in the facility on the way. Eventually after drifting in space for a while she was picked up by a ship where the crew used her (implied rape) before selling her into slavery.
From here on out Jack lived a life of crime and extreme violence while being haunted by her past, her crimes included things like piracy, joining a cult, and even the destruction of a space station - until her eventual capture and imprisonment on the Purgatory prison ship, a prisoner ship run by The Blue Suns where prisoners were frequently abused, here Jack was put into Cryo-stasis after proven to dangerous to keep around both to herself (she was worth a lot of money to the right buyer) and others.
And there she remained until the day Shepard came to the ship under the instructions of the Illusive Man with the intent of recruiting Jack for the "Suicide" mission.
Jack was created by Bioware as a companion character and possible romance option for Commander Shepard in the Mass Effect game series. She is voiced by Courtenay Taylor.
Powers and Abilities
Biotics - Jack is one of the most powerful biotics in existence, biotics is used a lot like telekinesis and grants the ability to levitate, lift, throw, move objects as well as the ability to create force-fields. It can also be used as a blast power.
Marksman - Jack is also very adept at the use of weaponry, especially Shotguns & Pistols.
Mass Effect 2
Jack plays a big role in the events of Mass Effect 2 as one of the main crew members in the game. She is a potential love interest to the player character (Commander Shepard) and has her own side mission.
Mass Effect 3
Jack had a smaller role in Mass Effect 3 then she did in the second installment but she does have several appearances during the game. She can no longer be romanced but if you romanced her in the second game it carries over to the third.