Although rules for the National Hockey League differ from European and international hockey in some ways, the NHL is widely considered the premier hockey league in the world, so we'll take a look at the NHL rules.
Hockey is played on a sheet of ice 200 feet (61 meters) long by 85 feet (26 m) wide. The nets are 6 feet (1.8 m) wide by 4 feet (1.2 m) high. The puck is a disc of vulcanized rubber 1 inch (2.5 cm) thick and 3 inches (7.6 cm) in diameter. It weights about 6 ounces (170 grams). Pucks are actually frozen before the game and kept in a cooler so they don't bounce as much when they're in play.
The 60-minute game is divided into three 20-minute periods. If the teams are tied at the end of 60 minutes of play, a 5-minute sudden-death overtime period is played. During the playoffs, teams continue playing additional 20-minute sudden-death overtimes until one team scores.
Six skaters per team are on the ice at a time: a goaltender, or goalie, who stays on the ice for the duration of the game (barring injury), and five skaters who take rotating shifts that last from 30 seconds to two minutes or more. Usually, there are three forwards (left wing, right wing, and center) and two defenseman on the ice. In certain situations, some teams play with four forwards on the ice and one defenseman.
A hockey rink is marked by a red center line, which divides the surface into two halves of 100 feet (30.5 m) each. There are also goal lines running across the rink 13 feet (4 m) from each end. Sixty feet (18 m) from each goal line is a blue line, which marks the boundary of each team's defensive zone.
The nets are positioned with their fronts at the red goal line. To score a goal, players must get the puck into the opposing team's net. The puck must completely cross the goal line for the goal to count. It can deflect off of any rink surface, or any part of any player on the ice, including feet, prior to entering the net, and still count as a goal, with a few exceptions: If the puck is deliberately kicked in, or batted in with a hand, the goal will be disallowed. Also, the puck can't be struck with a stick above the 4-foot crossbar of the net.
There are two linesmen on the ice during a game. It's their job to call off-side and icing (see below). Two referees also man every NHL game. They can be differentiated from the linesmen by their bright orange armbands.
When a player moves into the opposing team's zone, the puck must cross the blue line before his skates do. If any attacking player has both skates across the blue line before the puck, off-side is called. This results in a face-off.
At the start of each period, and after stoppages in play due to goals, penalties or the puck leaving the rink, play is initiated with a face-off. The centers from each team face each other over a face-off location designated by the official, and the other skaters line up at least 15 feet (4.6 m) away. The official drops the puck directly between the opposing centers, who then vie for control using their sticks and feet.
Hockey has a second form of off-side, known as the two-line pass. As the name implies, a two-line pass off-side is called if a player receives a pass that has crossed one of the blue lines and the center red line before he touches it.
The last "line rule" in hockey is icing. This rule was put into place several decades ago to prevent teams from simply slapping the puck the length of the ice to waste time. If a player shoots the puck into the opposing team's zone from his side of the red line, and the puck crosses the goal line at the other end without anyone touching it, and an opposing player other than the goalie then touches it first, icing is called on the attacking team. This results in a face-off in the attacking team's zone. But if someone on the attacking team is the first to touch the puck, or if the defending team's goalie touches it first, the icing is waived off.