Board Thread:Questions and Answers/@comment-15858627-20140204223209/@comment-1038387-20140204224341

Before 1986, there were lots of different continuities (Earths), but the most prominent were Earth-One and Earth-Two. Both had a cousin of Supes: Kara Zor-El (Earth-One) and Kara Zor-L (Earth-Two). In 1986, Crisis on Infinite Earths merged all the different multiverse worlds into one, later named New Earth. Kara of Earth-One died in that cosmic event. Kara of Earth-Two survived.

After the Crisis, DC wanted to reboot Superman into the Last Kryptonian, so that spelled trouble for his cousin. Supergirl was reintroduced, but this time she was a pile of goop from another dimension. That pile of goop later merged with Linda Danvers. Who later turned out to be an angel. Or something. Meanwhile, Power Girl's Kryptonian origins were retconned away; she was apparently a descendant of Arion. A couple of other origins also arose.

Around the mid-noughties, DC decided it was getting too ridiculous. They brought back much of the Earth-One Superman, including Kara-Zor-El (New Earth). Kara Zor-L (Earth-Two) was firmly established as being from Earth-Two again.

The two Kara's now frequently work together. They're sort of sisters.