The first role is Subscriber, this user has permission to read. We are not employing the Subscriber role to site visitors at this time. Visitors can use the rss feed from MMM to subscribe.

The next role is Contributor, this user has permission to read, write their own posts, edit their own posts, and delete their own posts.

Authors have all the same privileges as Contributors plus the ability to publish and manage their own posts.

Editors have all the same privileges as Authors plus the ability to moderate comments, manage categories, edit and publish pages, edit and publish all posts, and read private posts. We are not employing the Editor role at this time, this may change sometime in the future.

Administrators have all the same privileges as Editors plus the necessary access to install and uninstall plugins, make changes to the site, back up the database, etc.

One role is not better than any other role, an author is not more important than a contributor, and an admin isn’t much good if authors and contributors don’t exist (because no matter how pretty a website, if there’s no content it’s not going to work.) Your role might shrink or expand during the natural growth of the site, and you might request a role with more or less privileges than the one you have now.

For a more detailed overview of the default roles visit the Codex article.