You can also get 2 points for a safety, when the offensive team commits an infraction or is tackled in their own end zone. In addition to scoring 2 points, the defending team also receives the ball via a free kick. They are rare, but there have already been a few this season.
So these are the possible ways to score:
- Touchdown - 6 points
- Field Goal - 3 Points
- Two Point Conversion* (following a touchdown) - 2 points
- Safety - 2 points
- Point After Touchdown* (field goal-style kick) - 1 point
- (NCAA, and NFL as of 2015) Blocked extra point try (or any turnover on a PAT or 2-point conversion) is run back - 2 points for defense.
- Extra point attempt that results in a safety for either side - 1 point
In the game you are referencing, there was a safety and then a field goal, resulting in a 5 point score for Seattle.
*A Two Point Conversion or a Point After Touchdown can only be attempted immediately following a scored touchdown. The team must choose to attempt one or the other.