Elgg basically recognizes two types of users: members (ordinary users) and administrators (users who can do everything on the site). More often than not, you'd need additional user types and associated privileges what each "user type" can do to your site.

By using this plugin, you can easily introduce custom roles, restricting groups of users to certain privileges.Please understand that the Roles plugin, in its current state, is rather a framework than a full stand-alone plugin. You'll need to make additional tweaks, or use extension plugins, to make the custom roles work.

You can find detailed documentation under docs/manual.pdf.