Angels, of course, have been around long before Supernatural and are mentioned in Judaism, Christianity, and Islam. They are servants of God who act as a link between Heaven and Earth, and more specifically there are various functions they are said to serve - from smiting the crap out of sinners to carrying prayers to God. Angels are often divided into different groups or hierarchies, the most common of which has nine orders with the highest closest to God and the lowest closest to man. Supernatural, of course, follows some of the more violent job descriptions and they're not so much the "angel on your shoulder" type.

Angels are said to never die, having been referred to as "incorruptible substances" (Guiley, p. 15). Although they have no form, they can appear in human guise or as brilliant beings of fire, light, or lightning. This was seen when Anna regained her grace and promptly exploded in a blinding flash. This does differ, however, from how most angels in Supernatural are unable to take human form without finding a willing host.

As for fallen angels such as Anna, they are often thought - like Lucifer - to fall because of pride, but there are also some who believe they can fall because of lust. Angels are supposed to be sexless (in the biological sense) so sex (in the biblical sense) is a ticket straight out of Heaven. Once they fall, it's commonly thought that they become demons, but it's also been said that they simply become corporeal - in other words, a fallen angel could become a human.

So what about Anna?
The question still remains of where Anna fits into angel mythology, as “Anna” clearly isn't the name of a real angel like Castiel or Lucifer. The consensus online seems to be that she is probably Anael, and I agree that's quite likely for a variety of reasons.

According to the angelic hierarchy proposed in the 5th or 6th century, Anael falls under the seventh order of angels, the “principalities.” The job of these angels is to "watch over the actions of the Earth's nations and cities - the visible world of humankind" (Guiley, p. 38), which ties into Anna's statement of how she had been stationed on Earth for two thousand years. Anael is also considered one of the chiefs of the principalities, which again makes sense when you think about how Anna used to be Castiel and Uriel's superior.

Random facts: Anael is the chief angel of Friday and governs over the planet Venus. (Ok, so that had nothing to do with anything, but we all learned something, right?)

So now let's all sit back and wait for season five to prove me wrong!