| m u s h i n |

Here is a good diagram, which I found while doing some research on the Catalyst Perl Framework :

Note the red-colored squares after user, which represent its roles. Basically, a role could be a way to view a certain data set with certain privileges. We sort of see that in certain DBMS system, but I think it should we way more outspread in Web applications.

From : http://blog.woobling.org/2009/06/users-accounts-identities-and-roles.html

Enjoy!