A community differs from groups in that anyone can join a community. You create a community by going to Your Network, Owned Communities, then create a community and when you post a blog entry from the community blog anyone can click on the logo and click..join this community.
You share common interests. The Community's blog shows all the blog entries every posted from that Community. Members of the Community can share ideas, files, and audio recordings that will become podcasts since they are able uploaded, published and syndicated.
Hope this helps everyone grasp the concept of communities.