Whilst you can create any number of specialist usergroups to give segments of your users special priviledges, there is one usergroup that everone is always a member of. This is the "Visitor" usergroup.
Any user, regardless of whether they are logged on, is a member of the Visitor usergroup, and typically a website will be set up so that the public-facing pages will give the Visitor usergroup View permission.
In addition to this generic Visitor usergroup, it is possible to creat extra usergroups, which behave in the same way, but tied to specific domain names. For example, a website could be set to listen on two domains: www.domain.com and www.domain.fr. In this case, it might be desirable to show pages in English to visitors to the .com domain, and a similar set, in French, to visitors to the .fr domain.