Summary for the Brits and others outside North America...
Denver had what was widely considered the best offensive team, Seattle the best defense. That should have guaranteed an exciting matchup, and is why this particular game was highly anticipated. Opinions were split as to who would win. A great deal of money changed hands in legal and illegal wagering. Even I had $1 at risk (which I lost).
So it was a great surprise when Seattle's defense outscored Denver's much-vaunted offense. Unlike your football, where only one player on each team performs a purely defensive role and players switch from offense to defense every few seconds, in American football each team assumes one or the other role at a time. The Defense is not expected to score points, and doing so even once during a game is a rare event.
It is probably also noteworthy that the two teams represent the only two states that have legalized marijuana. You may draw your own conclusions from that.