An enforcer or a noted "tough guy" known for his fighting skills. Goons are usually fourth line players that are limited in skill who see limited action during a game and usually do not play on special teams (power play or penalty kill).
Goons can also be considered as "energy players" because they can re-energize their team with a fight or a big hit. They can also be used by their coaches to intimidate opponents and get them off their game because they will be more cautious when they know that a goon is on the ice. Goons are also given the role of protecting their teammates, especially the star players on their team. Opponents will themselves attempt to get star players off their game by hitting them, so a goon makes sure to make his presence felt by opponents on the ice. Famous examples of goons being used as protectors for star players include Dave Semenko, who was seen as Wayne Gretzky's protector with the Edmonton Oilers, and Georges Laraque, who was acquired by the Pittsburgh Penguins to protect Sidney Crosby.
Recently, the number of goons in the NHL has been declining due to the decreased number of staged fights during games. Because many goons are primarily fighters with limited offensive or defensive skills, coaches stop using them and many of them get traded or demoted to minor hockey leagues when they are not needed.