In the simplest terms domain means a large network and work group implies a smaller network. Work group being inherently small network of computers (around 10-15), one can access only when they are part of the network.All these computers are in the same local network or subnet. In workgroup, the computers are all peers and equal, each computer has a user account and the person using it needs individual account for that specific computer.
Domain being a large structure, it can be accessed from anyplace as long as internet connectivity is ensured. There can be many computers, computers from different local networks and the number of computers are usually hundreds or its multiples.Whoever has access to the domain, can log on to any computer in the domain without needing a separate user account for that specific computer. This enables the person having domain access to make changes, implement security protocols all easily and all the changes made will reflect on all the computers. Among the many computers in the domain, one or a few perform the role of a server, via this server, the domain users can introduce changes.