Answer:
In workgroup, all computers are peers; no computer has control on another computer.
Each computer has a set of user accounts. To use any computer in a workgroup you should have the account on that computer. All computers must be in the same network or subnet.
In domain
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One or more computers are servers. Network administrators use servers to control the security and permissions for all computers on the domain. This makes it easy to make changes because the changes are automatically made to all computers.
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If you have a user account on the domain, you can log on to any computer on the domain without needing an account on that computer.
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There can be hundreds or thousands of computers.
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The computers can be on different local networks.
Added by DaviePCRepair:
Basically: A workgroup is a group of computers and users sharing resources. Each computer maintains a list of users and their rights on that particular PC. A domain is a group of networked computers that share a centralized directory database of user account information and security.
Resource: Guide to managing & Maintaining Your PC Sixth Edition by Jean Andrews Pg.541