An Angel is a spiritual being that serves Judgement. Angels live in Heaven, but can be kicked out for misbehavior. Panty and Stocking are angels who have come down to Earth to save the world from ghosts. The sisters receive orders from Garterbelt, a priest.
It is implied that they may be immortal, as Stocking stated that she has been on an 500-year waiting list for a certain dessert. She and her sister also have been seen not to age a bit throughout the whole series.
Angels are supposed to be the protectors of innocents from demons and other evils. Judging by the behavior of Panty and Stocking, it seems that Angels (as opposed to the Order-obsessed demons) promote chaos, but also freedom. This should be taken with a grain of salt, though, as Panty and Stocking may not be the best examples of Angel behavior (they were kicked out of Heaven, after all).