User Roles Explained in WordPress
Different users may have different capabilities, in wordpress, there are six pre-defined roles: Administrator (Only for Multisite), Administrator, Editor, Author, Contributor, Subscriber. The capabilities of pre-defined roles in wordpress list here.
1. User Roles Defined in WordPress
Manage their profile only and read posts and pages.
Subscriber capabilities included, and edit or delete their posts, but can not publish them. nor they can upload files.
Contributor capabilities included, and publish and manage their own posts, Author can upload files.
Author capabilities included, and publish and manage posts and pages, including other users' posts management, etc. but he/she cannot edit/install/upgrade plugins/themes, nor he can create users.
Editor capabilities included, and all the other admiinistration capabilities for a single site.
All the wordpress capabilities, but you cannot see this role if you distribute wordpress as a single site.
2. How to change the user roles
If you have the capabilities of changing the user roles, you can change the role anytime in Users -> All Users -> Edit, then select a new role for the user, Click Update User to take it effect.
Wordpress defines 11 levels to indentify the roles's capabilities. In the database, wp_user_level is defined in table wp_usermeta. but this feature is deprecated since wordpress 3.0, you should not use it anymore.
Capability vs Role Table refer to http://codex.wordpress.org/Roles_and_Capabilities