SharePoint Team Site with access for both Internal AD Users and External Non Ad Users (But not anonomous users)

I think the title says it all but what I m trying to achieve is that I have a team site that needs to be used by both intenal AD users and external Non AD Users. If the site is accessed internally then the users AD details are automatically used but if the site is accessed externally by non ad users then the user will be prompted for a non AD username and password. However the internally AD users should still be able to access the site externally using their AD credentials

Hope that makes sense.

Could some one point me in the right direction :)

Thanks

Paul