The DMZ (or de-militarized zone) is an integral part of firewall security. It is a network that is neither part of the untrusted network, nor part of the trusted network. Rather, this is a network that connects the untrusted to the trusted. The DMZ creates additional layers of security against potential attackers. A Google search on "network security" AND "DMZ" will turn up any number of valuable references and FAQs that will provide more information than you ever thought possible.
This was first published in November 2002