The NFL’s rulebook describes over 60 fouls that a player or a team can commit and so, like most sports referees are put in charge of the game to make sure nobody is cheating.
Each ref has a different responsibility and area to watch during each play. Any one of the referees can call a penalty. To signal a penalty referees will throw a yellow “flag” onto the field. And if you think about it, throwing a flag onto the field is a quick, visual way to alert everybody—players, fans, and other referees—that a foul has been committed.
After the play, the referees will huddle together and the ref who threw the flag will inform the head referee (the guy wearing the white hat) what the foul was. For most games, especially those on TV, the referee will then announce to the crowd what the penalty was and what the consequences are going to be.
He’ll say something like, “False start, number 65, offense, five yard penalty, first down.”
This tells us quite a bit. First ‘false start’ is the specific violation the team made, number 65 on the offensive team is the player guilty of committing the penalty, five yards is the penalty, and the next play will be first down.
For most fouls, the penalty is the loss of yardage, sometimes from where the foul is committed but other times from the previous line of scrimmage.
If a foul is committed during the play, the down will be usually be repeated. For example if it is second down and ten yards to go, and there is a holding call on the offense at the line of scrimmage, the offense will be penalized ten yards, so the line of scrimmage will be moved back ten yards, and it will be second down and 20 yards to go.
Here are some common fouls that you will see throughout the course of most games: