A configuration of a web server that appears to clients as an independent server but which is actually running on a computer that is shared by any number of other virtual servers. Each virtual server can be configured as an independent website, with its own hostname, content, and security settings.
DNS maps the hostnames of all virtual servers on one physical server to its IP address. The web server software then uses the "Host" header in the HTTP request to determine which virtual server the request was for, and then processes the request using that virtual server's configuration.
Virtual servers allow Internet Service Providers to share one computer between multiple websites while allowing the owner of each website to use and administer the server as though they had complete control.