Unless you’re the sole user on your WordPress site, chances are that you want to limit the permissions that other users have, so that you can use WordPress as a fully functional Content Management System (CMS). As the administrator, you essentially have access to everything, however, you may have users that you only want to be able to write articles. And that’s where WordPress user roles come in.
In WordPress, there are six user levels by default (if these don’t meet your needs, you can create custom user roles with a plugin like User Role Editor). By name, they are Super Admin, Administrator, Editor, Author, Contributor and Subscriber.
In summary, Subscribers are merely members – they can read articles and that’s it. Contributors have the added ability to create, edit and delete their own posts – understand that they can create posts, but not publish them (they need to be reviewed and published by an Author or higher).
Authors can publish posts, upload files and edit or delete published posts. Editors have much more expanded capabilities including all permissions regarding publishing and editing posts and pages, comment moderation and managing categories and links.
Admins of course have a much more expanded capability including the ability to install, edit and manage themes and plugins, manage users, manage the site settings and update the core WordPress files. Obviously, with such unrestricted ability, you want to truly consider who you grant Admin access to.
Then there’s the Super Admin, which is a new user role as of WordPress 3.0, which addresses the multisite component of WordPress. Super Admin is the person in charge of a network of WordPress sites and has the ability to manage the network and its sites, including the users, themes and options.
The WordPress Codex has a very detailed list of exactly what each user role is permitted to do, but this article should give you a good idea. Have you got a site which makes good use of several of the user roles?