The group "*" is automatically given to all users. Users, who are not logged in, are in this group (and in this group only). The group "user" is automatically assigned to all users, who are logged in. Since user group permissions are cumulative, these users will then have all permissions of group "*" and additionally those, which are assigned to group "user".
Manual:User_rights#List_of_permissions is a list of all possible rights, which a user group can have. Which of them you want to give to e.g. the group "user", depends on what ordinary users should be able to do. See Manual:$wgGroupPermissions for details on how to assign permissions.
Which rights which group currently has can be seen in your wiki on the page Special:ListGroupRights. The rights, which are assigned by default (meaning without you assigning anything special yourself), already are more or less fine for most wikis.
As an example, if your wiki is publicly accessible and you then assign the group "*" the right to edit pages and to upload files, this will be a guarantee that you get spam en masse: In form of spammy pages and in form of trash uploads. You should better not do that. ;-)
This post was posted by 88.130.109.101, but signed as 88.130.118.211.