It's the new (bad) way of doing things on the Web... On invite only. But Flock did not implement it so well and it's incredibly easy to be registered w/o being invited...
Without at all looking at the contents of the private web site, and looking only at the screenshots provided by Bart and the prose on the public web site (in particular this picture)I would say Flick is a bundle of Firefox and a specific community-based aggregator. The registration process looks like Orkut's, and apparently it provides help for blog posting. So I guess Flock becomes the most recent tool to integrate Mozilla editor!
To summarize, and I happily bet a cookie on that: a standalone, firefox-based, community-based browser and rss aggregator allowing you to blog (probably with WordPress) and directly notify Flock - and your communities - of your post.