On the surface, the NFL is a pass-fail league. Teams pass when they win, fail when they lose and should probably get an "incomplete" when they squeeze out a tie.
In reality, things aren't that cut and dry. There is almost always a middle ground on the grading scale.
In Week 6, for example, the Pittsburgh Steelers pulled off a remarkable comeback victory, but their win wasn't perfect. The Buffalo Bills lost a sloppy game, but there were still positives to take away from it.
The Oakland Raiders... well, OK, the Raiders deserve a failing grade for their embarrassment against the Seattle Seahawks in London.