A workgroup is an interconnection of a number of systems that share resources such as files &printers without a dedicated server .Each workgroup maintains a local database for user accounts, security etc. A domain, on the otherhand is an interconnection of systems that share resources with one or more dedicated server, which can be used to control security and permissions for all users in the domain. Domain maintains a centralized database and hence a centralized management of user accounts, policies etc are established. If you have a user account on domain then you can log on to any system without user account on that particular system.