Manual:$wgGroupPermissions

Details
$wgGroupPermissions is a two-dimensional array indexed by user group and available permissions; for example, " " allows registered users to edit. These groups can be assigned to users through the wiki Special:Userrights interface.

For in-depth documentation, see Help:User rights.

Extensions
Extensions such as RenameUser or MakeBot add new rights which can be configured and assigned in the same manner.

Default values
In the interest of completeness, this section details the default values used in all versions of MediaWiki. For the latest defaults and more info, visit (see Help:User rights).

Note that it is incomplete - so far it only goes as far as 1.6.0

1.5.x
These were the default values from 1.5.0 to 1.5.8: