You are looking at the documentation for the 1.3 version of the Apache HTTP Server, which is no longer maintained, and has been declared "end of life". If you are in fact still using the 1.3 version, please consider upgrading. The current version of the server is 2.4.
The term Virtual Host refers to the practice of maintaining more than one server on one machine, as differentiated by their apparent hostname. For example, it is often desirable for companies sharing a web server to have their own domains, with web servers accessible as www.company1.com and www.company2.com, without requiring the user to know any extra path information.
Apache was one of the first servers to support IP-based virtual hosts right out of the box. Versions 1.1 and later of Apache support both, IP-based and name-based virtual hosts (vhosts). The latter variant of virtual hosts is sometimes also called host-based or non-IP virtual hosts.
Below is a list of documentation pages which explain all details of virtual host support in Apache version 1.3 and later.
Folks trying to debug their virtual host configuration may
find the Apache
-S command line switch useful. It
will dump out a description of how Apache parsed the
configuration file. Careful examination of the IP addresses and
server names may help uncover configuration mistakes.