Great article about the basics of NFL defenses: 4-3 vs 3-4, Tampa-2, Zone Blitz, 3-technique tackles, etc. It comes from a Fantasy Football website, so there are some IDP notes interspersed throughout the article, but really - like the author says, "Take the time to learn a bit about defensive football. The effort will make your Sunday afternoons even more enjoyable."

Here's a FO article that works hand-in-hand with the "Ultimate Guide": Defensive Line Basics