When a computer is joined to a Windows domain, all sorts of things happen. The most important ones:
- User accounts in the domain become valid users on the system and can logon to it (unless restrictions apply).
- Domain administrators acquire administrative rights on the system.
- The computer itself gets an account in the domain, and uses it to authenticate against other computers.
- Local user accounts remain active and can stil be used to logon to the system; they're not recognized by any other computer in the domain.
- The computer name gets registered in the domain DNS (if it supports dynamic updates, which it should).
- Group Policies defined in the domain and targeted to computers affect the system.
- Group policies defined in the domain and targeted to users affect any domain user who logs on to the computer.
When the computer is member of a domain but can't connect to a domain controller, it can't validate user credentials, so any domain logon is going to fail; the exception is the last logged on user, which is by default cached and remembered, and can still succesfully logon. So, if the last logged on user was DOMAIN\UserA, a disconnected logon with the same user account will succeed, but a logon with, say, DOMAIN\UserB will fail. (This behaviour is configurable via policy).
Group Policies remain applied even in a disconnected scenario.