The Members plugin is a user, role, and content management plugin. Its purpose is to make WordPress a more powerful CMS by giving enhanced control over site users.
The current components are (more will come in future versions):
Edit Roles: Edit your user roles and their capabilities.
New Roles: Create new roles for use on your site.
Content Permissions: Adds a meta box on your write post/page editor that allows you to restrict content to specific roles.
Widgets: Adds a login form widget and user-listing widget that you can use in any widget area on your site.
Shortcodes: Creates shortcodes that you can use to restrict or allow access to certain parts of your posts and pages (or any other shortcode-capable area).
Template Tags: New functions for use within your WordPress theme for various things.
Private Blog: Allows you to create a private blog that can only be accessed by users that are logged in (redirects them to the login page).