CNN has an interesting article on firewalls that should give noobs an idea of how they work and what they are.

In part:
"February 27, 2002 Posted: 8:48 a.m. EST (1348 GMT)
By Robert L. Hummel
(IDG) -- A personal firewall is technology that helps prevent intruders from accessing data on your PC via the Internet or another network. It achieves this by keeping unauthorized data from entering or exiting your system."

Full story: