In the NFL, if the score is tied at the end of the fourth quarter, the game goes into overtime (OT).
The current overtime rules follow a modified sudden death format. In this format, the first team to score wins the game, unless a field goal is scored on the first possession of overtime. In that event, the team that did not score is allowed one possession to try to score.
Overtime is a 10-minute period. (Before 2017, overtime was 15 minutes.) During the regular season, if the entire 10-minute overtime period is played and the teams are still tied, the game ends in a tie.
During the playoffs, however, tie games are not allowed. If the overtime period ends with no result, another overtime period is begun, and the game will continue until there is a winner.