Manual:$wgGroupPermissions

Details
$wgGroupPermissions is a two-dimensional array indexed by usergroup and available permission. The value can each time be either TRUE to grant the permission or FALSE, if it should not be granted.

Those permissions, which are granted with $wgGroupPermissions are always cumulative: If a user is member of different groups, then he will get a certain right, if it is granted to at least one of these groups - also, if it is not granted to his other groups. Or with other words: If one of the user's groups got a right granted, then - using $wgGroupPermissions - it is not possible to take the right away by not-granting it to another group. Instead use $wgRevokePermissions to revoke permissions.

Example
This gives the users of user group "user" (that are all registered users) the right to edit pages.

Custom user groups
You can also define your own user groups; user group names can be no longer than 16 characters. The groups, which have been defined that way (either in DefaultSettings.php or in LocalSettings.php), can be assigned to users through the wiki Special:Userrights interface.

Example
This creates a group called "Trustworthy". Users of that group have the same permissions as users from the group "user". However, they will be able to delete and protect pages, to patrol edits and to directly purge cached pages.

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

Default values
Default values vary from version to version. You may find the one which apply to your mediawiki setup in DefaultSettings.php. For more info visit Manual:User rights.

Use by extensions
Some extensions, such as RenameUser or MakeBot, add new rights which can be configured and assigned in the same manner.