The rules for regular season games are as follows:
If the teams are tied at the end of the 4th quarter, the games goes to overtime. Before overtime starts, there is a coin flip. The winner of the coin flip decides whether to kick or receive.
The first team to score in overtime wins, even a field goal is good enough.
If after the 15 minute overtime period, neither team was able to score, the game ends in a tie.
This would not be possible in the playoffs as there would have to be a winner so I guess it would go into double overtime (?)