Tasha grew up on the lawless planet of Turkana IV, learning to fight to avoid rape gangs before being rescued by Starfleet which inspired her to join. After graduated the academy with high marks in martial arts and tactics, Captain Picard, moved by her heroism called in a favor to have her on the Enterprise after he witnesses Yar rescue a civilian from a minefield
Tragically on a away mission to recover Counselor Deanna Troi who crash landed on the planet Vagra II were the evil black liquid alien Armus has been abandoned. In a show of power Armus kills Yar. As Guinan put it, it was a pointless death without meaning. She was succeded by Lt.Worf in the position as ship's Chief of Security and Tactical officer.
Denise Crosby played Tasha Yar, as well as Tasha's daughter, on the Star Trek:The Next Generation television series. She was orignally cast as Troi & Marina Sirtis was cast as Yar, but the two actresses had their characters swapped at the last minute.
In a alternate timeline where Tasha still lives and the Federation/Klingon war is still going on the Enterprise D encounters a time portal with the Enterprise C in side, This reality was created by the absence of the Enterprise C on a mission to help defend the Klingons from a Romulan attack, Tasha volunteered to go back to set things right. It is later revealed in the main time line that she and some of the crew became prisoners, to save the lives of the others Yar agrees to become a Romulan general's consort.
While performing this roll, she became pregnant and gave birth to a Human/ Romulan hybrid daughter named Sela. When trying to escape with her, Sela cried out alerting the guards, Yar was swiftly caught and executed.
Myriad Universes - The Last Generation
In this alternate universe, Tasha is alive and kicking, both involved with Ro Laren and a member of Picard's Resistance movement.
In her relationship with Ro, she is obviously the slightly more pessimistic one, a switch on perspectives from our universe. Tasha also seems to be friends with Wesley, rather then a mentor and have a nice rapport between them in the opening issue. Unfortunately, although understandably, that relationship immediately dissolved when he gets Ro killed in his childish mutiny he stages because he's angry with Picard.