It is essentially a derivative of Mozilla Firefox with a lot of customizations (and much better default theme). For a long time it's had 'built in' support for various services and accounts such as Photobucket, Facebook, Blogger, LiveJournal, YouTube, etc; I can upload photo's to my PB and post to my LJ without visiting the websites lol.
Flock 1.1 has added integration with Google and Yahoo's webmail offerings, if any one uses Firefox 2.x and either webmail, y'all might consider giving it a try.
I probably have access to a gmail and a yahoo mail account some where but I use Ippimail so it's not very important to me. I originally tried Flock because I wanted to try the del.icio.us and ma.gnolia support it has built in (i.e. storing my bookmarks online)
I don't really use most of flocks features, I'm just happy it has good performance on my laptop lool. Since I know there are several GMail and Yahoo users out here I thought some one might be interested in it.