A role gives users permission to perform a group of tasks. When a user logs in and is authenticated, the user's role determines which capabilities the user has, and each capability is permission to perform one or more types of task. All users with the same role normally have the same capabilities. For example, users who have the Author role usually have permission to edit their own posts, but not permission to edit other users' posts. WordPress comes with six roles and over fifty capabilities in its role-based access system. Plugins can modify the system.

Reference http://codex.wordpress.org/Glossary