This site contains articles written by several fairly knowledgeable people, including one who has been a professional scout in the NFL. You can get a decent education here on how football works.
A Day in the Life was written by an offensive lineman, and it's a story about a day in the life of the trenches of football.
If you're completely new to football, start with Players and Positions, which is an entry level introduction to who is on the field and what they're supposed to be doing.
The I Formation, Diagrammed Plays, and the History of the WCO are about how the offense works.
The Draft, Free Agency and the Salary Cap are about how General Managers build football teams.
Knees and Ligaments is a quick overview of how your favorite player just got hurt, and what is likely to happen to him.
The Rules is to settle bets, I suppose.