@indigo I looked at the responses others are giving you and I'm gonna try to take a slightly different vein here.
You post microblog messages to your own site. Your site has RSS so others can read them, right? micro.blog's main site acts like a 'social RSS reader'. You see a feed of RSS of all the people you follow.
Then, you can reply to those people right there on m.b
What happens here is one of two things -
a) either they have special plugins installed to 'pull' your comments back to their sites. This will show your comments on their sites and their comments on yours, if you have those plugins installed too.
b) if you don't have those plugins, it doesn't matter. You can just have a conversation with those people on microblog's site.
In that sense, it would be if on 10Centuries, we were posting all original messages on our own sites and then coming to Cappuccino or Mocha to do follow-up comments.
I hope that explains m.b better!