So, here's the complicated version. Back in the 60's, there were two completely different football leagues, the AFL, and NFL. In 1970, they merged to become the NFL with 2 conferences inside (AFC, American Football Conference, and NFC, National Football Conference). So, at this point, they went from 2 leagues, to 1. Over the years, more teams got added, and with the newest addition of the Texans in 2002, the current system was formed. In each conference, there are 4 divisions based on geographical differences (north, south, east, west). Most teams follow this, but there are stupid exceptions like the Dallas Cowboys who are in the NFC east. Now the top team in each division gets an automatic playoff spot (and depending on their record, will be associated with the right seeds). Now, there are 8 teams in all. But, there's something else. In each conference, the top 2 teams get byes, and the next top 2 teams (not counting division winners), go to the playoffs. In the playoffs, there is the first round, which is the wild card. Next is the divisional round, where the teams who had the byes now play. After this is the conference round, then the final super bowl.
This may seem really confusing, but it's not too bad. First you have to take into account that even though there are 2 conferences, it's the same overall league. There are interplay games between the conferences all the time. The NHL and NBA work the same way. There are 2 separate conferences (this time east and west) in the same league. Both leagues have 16 teams in the playoffs, and no teams get byes. There is also interconference play all the time. Now, the MLB is really weird. There are 2 separate leagues in major league baseball, and they are really different. Idk why. It just is, and I think it's completely stupid. Interleague play is pretty rare, and there's really weird arbitrary rules each have. Idk how MLS works, but more people here watch the Premier League than it haha.