Permissions to do things: Access Control
There are different levels of permissions for doing things in a Joomla! Web site. In particular there is a distinction between:-
Which users can gain access to what parts of the website? For example, will a given Menu Item be visible for a given user?
What actions a user can perform? For example, can a user edit or publish an article?
Users are grouped together in groups. In Joomla! 1.5 these groups give robust but fairly simple distinctions. It has a coarse granularity in that you cannot set permissions to small groups or individuals.