User allows you to build very powerful and complex member areas on your site. By default, ExpressionEngine comes with a special area (unlike regular EE templates) for member templating, and is very difficult to customize and impossible to stray from the structure in place. User slices up all that functionality into regular EE template tags that you can use wherever you want, however you want. Best of all, User only extends the EE Member module, so User becomes a plug and play add-on that works with existing Member data, and if you ever want to stop using it, can revert back to EE member module approach, etc.
User is also capable of much more! It brings to the table a robust set of front end member management tools and adds much more flexibility to the way your member management behaves. For example, you can send an automated custom welcome email to users once their registration is complete, Admins can register new members via the front end, create invitation emails with unique registration keys, set the email address to be the username for logging into your site, Custom error pages... and more!
User also includes a User Roles feature that allows you to create, manage and assign roles to users, restricting blocks of content, forms, admin tools, viewing of channel entries, etc to users with sufficient privileges on the front end EE templates. Also included is a Related Authors fieldtype that allows you to relate/assign members to channel entries.