it's called the Targa Graphic format. One program you can use to view these images is IrfanView (Google it, should have no issues finding it).
I've noticed that screencaps taken during certain game sequences, such as warping in or out, get captured in TGA rather than the standard format. Don't know why, maybe someone else can answer that part for ya.
But go get a copy of irfanview and you'll be able to view those TGA's just fine. And I think it will convert them to your favorite format as well, so you can always convert them to JPG for easier viewing.