Apache’s NameVirtualHost mechanism allows (among other things) a single numeric ip address to handle multiple domains’ websites. For example, the DNS entries for both this site (home.acheson.org) and another site point to the same numeric ip address. Apache delivers the appropriate content to incoming requests based upon the domain name in the request record.
Setting this up is quite straightforward and is well described in Apache’s documentation: http://httpd.apache.org/docs/2.2/vhosts/examples.html