I found this little write up about firewalls at How Things Work, a great site. Find it here. Very well put together, very informative, and it's not too dry reading material. I don't know about others, but over a period of time, I learn about things in bits and pieces, so eventually I may understand the basics of some subject, but still have some holes in the knowledge bank. This article should fill in any holes for someone that is just learning about firewalls or knows a little about them.