To elaborate on TerribleTrike's explanation, Powergirl was originally from the next universe over from the mainline DC universe that was destroyed in one of the Crisis mega crossover events to show how high the stakes were.

Powergirl wasn't in her home universe when it was destroyed, so when the dust settled she stayed in the mainline DC universe because she didn't have anywhere else to go.

So that's why there are two Kara's running around, and it's also why they're different ages leading to Powergirl being noticeably more… developed… than Supergirl (Because the timelines of the two universes didn't quite line up).

Powergirl is Supergirl from a parallel universe, and is therefore a Kryptonian as well.