Each component in the site can have its own set of permissions, and these can be set differently for each usergroup.

If the permissions are not explicity set, they will be inherited from the permissions set in the parent section (and if not set there, then set from further up).

Ultimately, at the top of the site, some basic default permissions are set, that allow visitors to view pages, and administrators to edit them. However you should always review the permissiosn for your sepcific requirements and set them appropriately.