Questions relating to virtual hosting; that is, serving content for multiple logically-separate entities from the same machine. Usually relates to name-based virtual hosting in the web server, but can also cover other protocols such as SMTP or IMAP.
HTTP Virtual Hosting
In HTTP, virtual hosting refers to the serving of content for multiple domains from a single server. The most common form of virtual hosting is properly called "name-based virtual hosting", where the content to be shown is based on the domain name being requested. Less frequently, virtual hosting can be done by using the destination IP address to select which content to show (this is most commonly used when hosting HTTPS websites).
Guides for configuring virtual hosting in a number of common web servers include:
Mail Virtual Hosting
Mail service can also benefit from a form of virtual hosting. In this instance, the receiving SMTP server performs local delivery based on the entireity of the e-mail address (rather than just the local-part), and users login to their POP3/IMAP server using a username that includes their domain (this is usuallythe user's e-mail address) rather than a bare username, and the server looks up the login details and configuration information for the user based on that fully-qualified name. This allows multiple users with the same local-part to use the same server.
Most "integrated" mail services support this feature "by default"; that is, users always use their full e-mail address to login. However, most standalone mail servers do not support this by default and require additional configuration.