Users, Roles, And Capabilities in WordPress

What is the User system for?

The User / Roles / Capabilities system controls what people can do and provides a framework for storing and using information about those people

Terms

Users: A registered entity with a unique login
Capability: The right to perform a type of task
Role: A group of capabilites that are assigned to a user*

Users can be assigned into multiple roles, though I discourage it. Users can also have capabilites given and taken specificly from them and not messing with the role

Cautions

Users can be assigned multiple roles
Capabilites change and are added each version
Don't think of one role being supperior to another

Default Roles

Subscriber: Can Read
Contributer: Can write or delete, but not publish, their own posts and also read
Author: Can write, delete, and publish their own posts, but not others. Can also upload media, and read
Editor: Can Write, delete, and publish everyone's posts and also moderate comments, manage catagories and links,
Administrator: Whatever they damm well please

How do you check a capability?

if ( current_user_can('capability') ):
	//do something
endif;

Check wp-admin/includes/schema.php for the most up to date list of default capabilites