Because football is a high-contact sport requiring a balance between offense and defense, many rules exist that regulate equality, safety, contact, and actions of players on each team. It is very difficult to always avoid violating these rules without giving up too much of an advantage. Thus, an elaborate system of fouls and penalties has been developed to "let the punishment fit the crime" and maintain a balance between following the rules and keeping a good flow of the game. Players and coaches are constantly looking for ways to find an advantage that stretches the limitations imposed by the rules. For example, in 2016 the Baltimore Ravens had all of their offensive linemen commit holding penalties in order to allow the punter to keep possession of the ball so time would expire for a win, since the game can end on offensive penalties. However, the NFL changed the rules after this to prevent teams from manipulating the game clock. The frequency and severity of fouls can make a large difference in the outcome of a game as well, so coaches are constantly looking for ways to minimize the number and severity of infractions committed by their players.
It is a common misconception that the term "penalty" is used to refer both to an infraction and the penal consequence of that infraction. A foul is a rule infraction (eg. offensive holding) for which a penalty (eg. move back 10 yards) is prescribed.
Officials initially signal fouls by tossing a bright yellow colored flag onto the field toward or at the spot of the foul. Because of this, broadcasters and fans often use the terms "flag" or "flag on the play" to refer to fouls during the game.
During a play, multiple officials may flag the same play, and multiple flags may be thrown for separate fouls on the same play. If applicable, the same official can signal additional fouls on a given play by throwing a beanbag or their hat. When officials throw a flag during a down, play does not stop until the ball becomes dead under normal conditions, as if there were no fouls.
Once the ball is dead, or immediately when a foul is called after a play is over or prior to a snap (since the ball is dead anyway), the referee, the official(s) who threw the flag(s) and other officials with a view of the play confer to come to a consensus on whether an infraction was actually committed, what it was, and who committed it. The final determination and assessment of the penalty is the sole responsibility of the referee. The referee then makes initial visual body signals to the press box indicating what fouls were committed and the team that committed them, the latter shown by extending the arm toward that team's end zone.