Salesforce Sites guest user characteristics

1. Each site has a special user with a user type called guest user type

2. Guest users have no password, no role and have no API access on their profile.
3. You can't login or do client side api call (only within apex code).
4. There is no session for guest browsing
5. Each guest user (site) has a unique profile (public access settings)
6. These profiles are associated to guest user licenses which give the ability to read/create on standard objects and full CRUD on custom objects. However, by default no CRUD is given. Admins need to enable the right level of access based on their flow
7. We advise private sharing for the objects that are enabled for sites
8. Guest user licenses can't be used for any other purposes other than Force.com Sites.
9. Custom Profiles with Guest User License cannot be added to any Metadata API, such as Changesets, Force.com IDE or ANT. For more, please review Guest Profiles Can Be Migrated and Created Through the Metadata API. 

See Also: 
Sites and Site.com User Licenses