Boarding in hockey is a physical penalty called against a player who aggressively pushes or checks another defenseless player into the boards that outline the rink. For a player to be defenseless, they either have their back to the player and are not aware of the incoming hit or are not reasonably close enough to make a play on the puck. Boarding is often called when the offending player has no direct intentions of getting the puck and is called regardless of if the hit was intentional or accidental. Players are restricted from hitting any player this way who is in a vulnerable position or from hitting anyone into the boards as a means of intimidation.
Hockey is quite a physical sport, in fact, one of the only sports that allows some fighting, but there are rules that govern the physical nature of the sport. This rule is one of the many physical penalties in place to ensure players' safety. Players are allowed to check in order to get the puck free, but checking someone into the boards on the ice can prove to be quite dangerous. The player checking is responsible for making sure the check does not put the opponent in danger.
The boards that surround the ice can be made of wood, plastic, or fiberglass; traveling at high speeds and getting hit into any of these materials could result in injury, even with all of hockey's protective padding. It's especially dangerous when a player is defenseless and does not see the hit coming.
The consequences of boarding vary on the severity of the hit. At the end of the day, it's up to the referee to decide what the punishment should be for boarding. In some instances, when players appear to know the hit is coming, a boarding penalty might not be called. On the flip side, a boarding penalty might be called for a player who wasn't necessarily hit that hard, but was defenseless and/or vulnerable. Boarding can result in a minor penalty, a major penalty, a major penalty plus a game misconduct penalty, or a match penalty. Minor penalties are the least severe, then major penalties, then match penalties. If the boarding penalty results in an injury, the player that committed the penalty will get a major and a game misconduct penalty. If a player commits a boarding penalty with the intent to injure in a opponent, he will receive a match penalty. Some boarding penalties could even result in suspensions or fines. If a player incurs two game misconduct penalties, he receives a suspension for one game.
Boarding is often confused with checking, which is a violent move toward another player by skating or jumping into them. It's very similar to boarding, but the key danger with boarding is the involvement of the boards in the hit. Among other physical penalties are: clipping, elbowing, head butting, illegal check to the head, kicking, kneeing, slew-footing, roughing and throwing equipment.