A hockey game is divided into 3 periods of 20 minutes with a ten-minute break in-between. If the game remains tied after 60 minutes, there are 5 minutes of 3 on 3 overtime. If the game is still tied, there is the shoot-out.
A hockey game always has 5 skaters + a goalie per team, but in overtime, it is 3 skaters + a goalie per team. This means much more open space and high-intensity chances for either team.
A shootout is much more different. Each team sends one player at a time to score on the goalie and after three tries, whichever team has the most goals, wins. If it is still tied after that, then they trade chances until one team scores and the other misses.