A comic book character, owned and published by DC Comics. She was first introduced in All-Star Comics #58 back in 1976 and was created by writer Gerry Conway and artists Ric Estrada and Wally Wood.
There have been two different versions of Power Girl. During DC's Pre-Crisis era, when there were a whole bunch of parallel universes, Power Girl was the Supergirl of Earth-2--her father was a Kryptonian named Zor-El, who was the brother of Jor-El, who was Superman's daddy. Like Supes, her parents sent her away from Krypton in a rocket, but her ship took the long way around, so she arrived on Earth much later than her cousin. After taking a secret identity as Karen Starr, she became a member of a superteam called the Super Squad, which was basically a junior Justice Society. And most importantly, because one of the artists was Wally Wood, one of the best creators of "Good-Girl" comic art ever, Power Girl was a hot blonde uber-babe who wore a low-cut white costume.
After the Crisis on Infinite Earths eliminated most of the parallel Earths and merged Earth-1 and Earth-2, DC decreed that Superman was absolutely, positively the last survivor of Krypton, so a new origin story was going to be necessary for Power Girl (who, as a hot blonde uber-babe wearing a low-cut white costume, was much too popular with the fanboys to be abandoned as a character). Though the new version of Power Girl started out believing that she might be from Krypton, it was eventually revealed that she was actually the granddaughter of Arion, a wizard in ancient Atlantis. She had been sent forward in time and given superpowers in order to help her escape from an evil sorceror named Garn Daanuth. Eventually, DC revealed that she actually is the pre-Crisis Power Girl from Earth-2, which makes her Superman's sort-of cousin.
Power Girl has been a member of the Justice League, the Justice Society, Infinity, Inc., and the Sovereign Seven. She is generally written as the owner of a technology and software company called StarrWare and owns a cat named Stinky.
Her best adventures have generally been written by Jimmy Palmiotti and Justin Gray, with art by Amanda Conner. Despite the fact that she really is a fun character, she doesn't seem to show up in comics very often these days.
Some research from http://www.toonopedia.com/powrgirl.htm