Can I use the same authentications of internal users (available in AD) to external users.?
If no what all authentication method I can use for external users?
Is there any difference in authentications from SP 2007 and 2010?
Is there any difference in authentication in SharePoint versions, that is foundation , standard and server.
SharePoint uses Zones to control access internally and externally. Authentication methods provide flexibility to authenticate users from inside and outside of a domain/enterprise network. However SharePoint also provides other functions to help manage authorisation and security of data (eg. Lockdown mode, which puts a site in readonly/access mode for internet access). Also, it allows people to access information with different security, based on authentication by extending web applications and having multiple authentication providers.
SharePoint uses standard ASP.NET authentication providers which means you can use SQL or other storage devices to manage users and roles using authentication.
You could use forms based authentication as well, which provides a login form within the browser to further extend authentication possibilities.
It is important to match security requirements with authentication providers.