When applied to a network of connected computers, the domain is not generally exposed to the outside world. A centralized serverknown as a domain controller authenticates users when they log on to their workstations. Resources with in the domain (other workstations,file servers, printers, access to the internet, email, etc) allow access to the user based on the approval of the domain controller. Microsoft'sterminology for central management in a domain is Active Directory.
Domain contruction involves determining a meaningful name (one that does not conflict with a well known public name), an adequate (ip)address range for the number of computers that will reside within the domain, setting up DNS (Domain Name Server) andDHCP (Dynamic Host Configuration Protocol) servers, creating accounts for all users, and connecting required resources (printers, file servers,gateways to the outside world, etc).
Domain structure can be as simple as a single server acting as a domain controller (Windows Server 2012 Essentials or Windows Server 2012),or much larger with other servers acting as backup domain controllers. Complex domains can have sub domains (Click here to see a graphicaldomain structure example). This type of domain structure might be required in a business with several semi-automomous divisions such asan accounting, engineering, and sales that are part of a greater organization. This structure is often referred to as a Domain Tree.
For information on a simple version of the Domain structure that does not require a domain controller, but is more management intensive,click on the Networking link above to learn about Workgroup networks.