A single Application Server installation on a server can encompass multiple instances. A group of one or more instances that are administered by a single Administration Server is called a domain. Grouping server instances into domains permits different people to independently administer the groups.
You can use a single-instance domain to create a “sandbox” for a particular developer and environment. In this scenario, each developer administers his or her own application server, without interfering with other application server domains. A small development group may choose to create multiple instances in a shared administrative domain for collaborative development.
In a deployment environment, an administrator can create domains based on application and business function. For example, internal Human Resources applications may be hosted on one or more servers in one Administrative domain, while external customer applications are hosted on several administrative domains in a server farm.
The Application Server supports virtual server capability for web applications. For example, a web application hosting service provider can host different URL domains on a single Application Server process for efficient administration.
For detailed information on administration, see Sun GlassFish Enterprise Server 2.1 Administration Guide.