Football has been, for all intents and purposes, a near monopoly by the NFL. It fended off about five different competitors but benefited from the innovations of some of them. Burt Reynolds and the USFL for example came up with the idea of cameras on wires floating above the field to give the kinds of action shots the modern game has. The AFL on the other hand gave us something much much larger.
Keep in mind that the competing leagues rarely got traction. Vince McMahon's XFL which he's attempting to relaunch this year lasted about one season. As did the CFL's attempt to spread its meters-not-yards three downs and you can kick for a rouge cancer southwards.
But the AFL was something different. It was a competitor for eyeballs and the best players with the NFL, and the two leagues had a near-equal rivalry for a while. But in the 1970s it was kind of agreed that the economics of the situation meant there could and should only be one league, and part of that was to have a merger, prior to which would see the champion team of the AFL fight the champion team of the NFL in a sort of super-competition to see which would be the best football team that year. Now that the NFL is a united league with NFC and AFC divisions, it is a matchup between the best AFC team and the best NFC team. So for example, there will never be a game between the Pittsburgh Steelers and the New England Patriots for the Super Bowl, nor will there ever be one between the New Orleans Saints and the Atlanta Falcons. This precludes a LOT of team rivalry matchups - it is a known fact that certain fan bases hate each other with a violent passion.
It's part of the rhythm of the football year that towards the end there is much angst and prognostication as to how and when teams can stay in or be blanked from the playoffs leading up to the Super Bowl. There are a lot of "if the Eagles and Saints both lose, and the Buccaneers and Falcons win, that means the Panthers are still in it, whereas if the Eagles and Buccaneers win..." scenarios played out by analysts and hung on to by fans hopeful that their team can somehow limp to the big prize.
It is usually announced, as well, when a team is mathematically eliminated from contention.
The winners of the conference divisions (NFC North, NFC South, etc.) are in, as are a couple of "wild card" teams seeded by a formula that takes in number of wins in the season but gives special weight to wins versus teams in their own conference. For example, the Buccaneers and Panthers might have an equal number of wins, but if the Panthers won against the Saints and Falcons and the Buccs won against the Patriots and Steelers, the Panthers would be seeded higher.
The divisional playoffs are usually violent affairs, but when the dust settles, there's one champion per division. The divisional championship games are almost as hotly contested as the actual Super Bowl itself.
The teams are given a couple of weeks off, and during the interim the football is kept alive in terms of interest by having the Pro Bowl (well, in theory anyway) - a game that invites the best players in the league (that because of the teams they're on they're not in the Superbowl) to a rivalry friendly by division in Hawai'i as a consolation prize. Truth be told it's a game of no consequence, it isn't played particularly hard (though it's an honor to be invited) and few people watch it. They also give out the awards for rookies of the year and so forth.
But then, on the first Sunday in February, at what is usually a neutral stadium (the stadium is picked over a year in advance, but no team has ever won a Super Bowl in its own stadium) the Big Game, as it is known by companies who haven't paid the exorbitant licensing fees to use that trademarked name kicks off.
To say it's an enormous event is an understatement.
There are Super Bowl events at bars. People hold Super Bowl parties. The advertising around the Super Bowl is insane. There are gallons of beer and soft drinks, acres of chicken wings, bushels of chips on display in bulk in stores urging people to stock up for the big day. Football themed cupcakes, balloons, you name it. People tend to have their own personal Super Bowl recipes, be it serving a mountain of food buffet style or cookout. The sheer number of calories consumed per American during the game is immense, even by American standards.
Church attendance in both cities in the Superbowl go way up, as people seek supernatural aid for their teams. Churches are happy to "cash in" by having things like food donation boxes, labelled for both teams, and you "cast your vote" for who you'd like to win by donating in the box marked with the team logo you want.
Of course, sales of big screen high definition televisions skyrocket (and returns of same the following week are not uncommon).
Electrical grids need to be on-point because of electrical demand, and given people's tendencies to use the toilet during commercial breaks, there's known drops in water pressure as entire cities practically flush in unison.
Regardless of whether your team made it to the final game or not, you're most certainly watching it. It's one of the few times a year where a casual bandwagon support of another team is allowed, but going out and doing a full team-swap transition is considered a huge no-no. Armchair coaching and quarterbacking will take place during the pre-game as men gather (and to be fair, many women) and discuss who's injured, what likely tactics will be, keys to the game for both teams and their strengths and weaknesses and so forth.
In fact, in some ways the game can be more fun. If your team isn't in it, it's not as much a nailbiter to see your team down or praying that your team holds the lead. You can actually enjoy the league's two best teams (well, in theory anyway) leave it all on the field and enjoy the game for what it is. Of course there's no sweeter joy than seeing your team win a Superbowl, especially if they're a franchise that has never won it (at the time of this writing that would be the Cincinnati Bengals, Buffalo Bills, Cleveland Browns, Arizona Cardinals, Los Angeles Chargers, Philadelphia Eagles, Atlanta Falcons, Jacksonville Jaguars, Detroit Lions, Carolina Panthers, Houston Texans, Tennesee Titans and Minnesota Vikings). This year the Eagles might win their first championship, so Eagles fans are flying high in Philadelphia.
But it's not as though it's just for sports fans. Because advertising rates are insanely high given the sheer viewing audience size, companies typically pull out the stops to produce either funny, insightful, moving or otherwise epic campaigns and/or spots for the event, which are often as enjoyed as the game and are as often commented on and judged as the sports played between them. Perennial advertisers Budweiser, Coca Cola and so forth are heavily represented but you also get quirky advertising by newcomers and hopefuls trying to make an impact.
Then of course, there's the half time show. It's usually a full-on abbreviated concert by a notable performer, these have included acts like The Who, Bruce Springsteen, Katy Perry and The Black Eyed Peas, complete with lasers, fireworks, and an A-game brought by the performer in question.
About 110 million people watch the game. That's one of the largest viewing audiences on earth.