Supergirl has appeared in different incarnations as DC Comics revised and reordered their fictional universes over the years. Kara Zor-El first appeared in Action Comics #252 in 1959, and has been a popular character ever since — even after she died and was replaced by other characters who took on the Supergirl mantle for a time.
Silver & Bronze Age
Post-Crisis: Linda Danvers
Post-Crisis: Kara Zor-El
Read more at Power-Girl.org
Power Girl is Supergirl’s counterpart from the alternate universe world of Earth 2. She’s a few years older than Supergirl and has a more cocky persona. Power Girl replaced Earth 2’s Superman when he retired from the Justice Society of America. In the New 52 universe Power Girl’s backstory has been revised (she is the former Supergirl of Earth 2) and she is teamed with Huntress (former Robin of Earth 2).
Kara Zor-El is Superman’s teenage cousin from Argo City, which survived Krypton’s destruction intact and protected by a barrier which maintained its atmosphere. When the city’s inhabitants began to fall prey to the deadly Kryptonite radiation emanating from their planetoid, Kara’s parents Alura and Zor-El constructed a rocket for their teenage daughter to escape their city’s doom. Kara reunited with her cousin Kal-El on Earth and took on the mantle of Supergirl. In time Kara reunited with her parents after discovering they were still alive, having been trapped by Brainiac with other Kryptonian survivors in the bottled city of Kandor.