I’ve always wondered about how these settings affect your level of access in Wordpress and experimented a little with them to see what it does, my findings:

Subscriber: A Subscriber can only read posts and comment on articles, if your blog doesn’t allow for comments from the public, you’ll need to be at least a Subscriber before you can post comments.

Contributor: The Contributor is a Subscriber that are allowed to post articles on your blog, although the posted articles will need to be approved by either an Editor or an Administrator.

Author: The Author is a glorified Contributor, Contributors contributing posts of such high standards that it doesn’t need approval becomes an Author and they can post without having to wait for an Editor or Administrator to approve their articles.

Editor: An Editor is an upgraded Author who can approve / reject / edit posts of other users as well as create pages.

Administrator: The Administrator is like having root access on a Linux system, they have all the privileges of the Editor, can manage users as well as change everything inside the control panel, the blog owner is typically the administrator.

When you set somebody’s role as “no role for this blog”, you basically cripple the account in such a way that the registered user becomes nothing more than a public reader.