Introduced in HTTP 1.1, a host header is a third piece of information that you can use in addition to the IP address and port number to uniquely identify a Web domain or, as Microsoft calls it, an application server. For example, the host header name for the URL http://www.ideva.com is www.ideva.com. An HTML 3.0 or later browser supports HTTP 1.1. The browser includes the host header name you specified in the location field of the request header that the browser sends to the server. If you don't specify a host header name in the request header, the root Web domain acts as the default Web server.

Learn more from "Using Host Headers to Set Up a Multihomed Server" and  "Handling Host Headers on Your Website."