Can't go into very great detail, but there is a discussion about choirs
here. They are, in my opinion, a way to classify angels by form and function. Different people subscribe to different structures... most widely recognized is
this one, and is the one I see as most fitting. Not every angel fits into just one choir, however... they may be of one in particular, but works so closely with members of another that they are considered part of both.
Angels serve God, the All, the Creator, the Source of life and reason. As for why we serve... we were created to serve His will, and it is our joy to do so. "Why" as in "to what purpose"... eh, just to keep life rolling all smoothly, interjecting and guiding as needed. There are other angelic-like creatures that fulfill similar roles to their own, separate deities, but allegiance to the Creator is a requirement to the title of Angel (or former allegiance, in the case of the Fallen).
Erhm... that's really just the questions above, and nothing else... sorry 'bout that, but I gotta dash early this morning <!-- s

--><img src="{SMILIES_PATH}/icon_e_smile.gif" alt=":)" title="Smile" /><!-- s

-->