Live Journal is like a blogging community website. You can sign up for free, or pay if you want more options and such, but the free one is good enough for most people.
Once you have a live journal you just kind of post to it. Its like being part a big community, people can post comments to your journals entires (if you allow that setting) and you can comment on other peoples journals.
You can add other people to your friends list, and place watches on journals you like. There are also tones of communities about every subject you can think of.
I have a live journal that I use pretty much for updating about my artwork for whatever fans watch it and such. I'm kind of in a web of other fantasy artist all of which watch each others journals
Im part of a few communities that interest me, including a couple hamster related ones, which all sorts of people can post to and share comments with each other.
Check my personal live journal, you can see how Ive been making posts and how other people have commented here and there.
http://crypticvitality.livejournal.com/
Now for an example of a community check out this link.. you can see how multiple people post to it.
http://community.livejournal.com/hamsters
I hope that kind of gives you an idea. If you make an LJ account be sure to join the new hamster central LJ community!