Microsoft Knowledge Base Article 313562 contains the following summary:
You can configure Microsoft Internet Security and Acceleration (ISA) Server to publish a Web server that is on an internal network or to use packet filtering. By configuring ISA Server to use packet filtering, Web requests can pass through to a Web server that is on a perimeter network, which is also known as a demilitarized zone (DMZ).
This step-by-step article describes how to use a tri-homed ISA Server to publish a Web server that is on a perimeter network. A "tri-homed" computer refers to a computer that contains three network adapters.