The entire league is split into two conferences of 16 teams each called the AFC and the NFC (American Football Conference and the National Football Conference).
From their, each conference is broken down into four divisions (North, South, East and West). So there is an NFC North, NFC South , NFC East and NFC West, and so forth with the AFC. Which teams are in these leagues are the same every year. Presuming the league doesn't make any changes or add any teams, these divisions remain the same every year, for example the AFC East consists of my team the New England Patriots, the New York Jets, the Buffalo Bills and the Miami Dolphins and has done since the league was re-alligned in 2002.
Firstly for 17 weeks there is a league system.
Each team plays every team in their division twice a year, once at home and once away, so that's 6 games. From there a team plays 4 other teams from the same conference who share a division, (so the
Patriots might play the entire AFC North (the Pittsburgh Steelers, Baltimore Ravens, Cincinnati Bengals and Cleveland Browns) which is 4 more games (two at home and two away). They then play 4 teams from the other conference in the same division (so the Patriots might play the entire NFC East (New York Giants, Philadelphia Eagles, Dallas Cowboys and Washington Redskins) so that's another 4 games which is 2 home and 2 away. The final two games (making it up to 16 games in total, spread over 17 weeks allowing for one rest or 'bye' week) are played against two teams in the same conference as them that finished in the same division position as them last year, but not in the divison which they played all the teams in as I put in bold. So for example, the Patriots finished 1st in the AFC East last year, and are hypothetically playing the entire AFC North, so they must play the Houston Texans (winners of AFC South) and the Denver Broncos (winners of AFC West).
Then the knockout stage begins. The AFC and the NFC have entirely separate playoffs systems until the superbowl (which acts as a final).
From there the 4 winners from each division (North, South, East and West) and the 2 teams with the best record who didn't win their division get seeded from 1-6 based on who won the most games. The first and second seed get a 'bye' round, with the 3rd seed playing the 6th at home, and the 4th playing the 5th. Naturally, the team with the higher seed gets the right to play at their home stadium meaning the number 1 seed gets 'home field advantage' because they can play every playoff game before the superbowl (which is held at a different stadium each year) at their stadium. Once the first round (called the wildcard round because the wildcards play in it) is over, they advance to the second round where the top 2 seeds are waiting, and the winners go on to the conference championship (effectively the semi-final), whereby only 2 teams remain standing in each conference, the winners of which become conference champions. Then, the AFC Champions play the NFC Champions in the Superbowl.
If that's too long, message me and I will make it shorter. Anything that is still confusing, I will explain again just ask