What is a firewall?
A (Internet) firewall is a machine which is attached (usually) between
your site and a wide area network. It provides controllable filtering
of network traffic, allowing restricted access to certain internet port
numbers (ie: services that your machine would otherwise provide to the
network as a whole) and blocks access to pretty well everything else.
Similar machines are available for other network types, too.
Firewalls are an effective "all-or-nothing" approach to dealing with
external access security, and they are becoming very popular, with the
rise in Internet connectivity.
For more information on these sort of topics, see the Gateway paper by
[Cheswick], below.