The users on your WordPress site are assigned to different roles. Roles determine what each user can and cannot do on your site. When you create a new user, you will need to assign a role to that user. You may also update a user's role in the future. The users described below are the default users defined in a typical WordPress installation. Some plugins may alter user role definitions, so be sure to check your plugin's documentation if you are unsure.
In order of greatest to least amount of power on a WordPress site, the roles are as follows:
Administrator -- Administrators have full access to the WordPress site. They can add, remove, and edit users. They can write, publish, and edit posts under their own name or for any other user. They can manage post categories and tags and can moderate comments. They are able to manage plugins, themes, updates, and site options.
Editor -- Editors don't have the site administration capabilities of the administrator (they cannot manage users, plugins, themes, updates, or options), but have the editorial power over everyone's content: editors are able to write, publish, and edit posts under their own name or for any other user, and are able to manage post categories and tags and moderate comments.
Author -- An author is able to publish, edit, and delete posts only under their own name.
Contributor -- A contributor can write and edit (including delete) their own posts under their own name. However, a contributor cannot publish any posts, and must await publishing from an editor or administrator.
Subscriber -- A subscriber is only able to manage his or her profile, and cannot create any posts.
Administrators and editors perform similar functions, but it's important to know when to assign each role. Both roles can publish, edit, and manage posts at-will, both for their user and for other users. However, only an administrator is able to manage the inner workings of the site: the plugins, themes, options, etc. Additionally, only an administrator is able to add, remove, and edit users.
Authors and contributors are both able to edit and write posts for themselves. However, only an author is able to publish a post to the live site without permission from an editor or administrator. Until a user has proven capable of writing appropriate and professional content, they should not be permitted to self-publish on your site.