Networking :: Network Security - How To Identify Programs And Processes

I was wondering if anyone might know of good reference material, books websites etc., that discuss network security issues in layman terms. I would like to set up a dedicated Linux box as a firewall and would like to have a deeper understanding of the different types of configurations that are possible. I run a dual boot system and most of the firewalls I have used on the Windows side are very confusing to me. A lot of the time they give you a pop up that informs you that some cryptically named program is trying to access the network or the internet and wants to know if I want it to or not, 99% of the time I have not idea if it is a legitimate program or not. I realize that this is probably a separate issue (knowing how to identify programs and processes that should have access from those that should not) from setting up a firewall and basic network security but I know that they are related.