Where can one find resources to learn about football defenses?

I have a really basic understanding of what the different positions do and what base defense designations like 4-3 and 3-3-5 mean. I've never played organized football at any level. I am ignorant as to what the fundamental philosophy differences between different defensive styles is, and what types of players are most important at each position depending on scheme. I'd really like to remedy that ignorance. I no-kidding read a little bit from the website for Football for Dummies to try to help

Does anybody have useful resources to learn these types of things? I have been Googling and found some apparently useful references (like the Football for Dummies one), but I'm at the stage where I'm too ignorant to know what's junk and what's not.

Thanks in advance for any help.