Pros and Cons of DMZ

[chrisp @ tidalwave . net (Chris Pressley)]

Assume a simple network, internal net, firewall, external net, Internet.
What are the pros and cons of putting a web server in a DMZ outside the
firewall. I understand (I think!) why I might want to put a web server
outside of a firewall, but why have a firewall with 3 network interfaces
rather than two?

Thanks for your help.
Chris