Ice hockey, the most popular form of hockey in most geographic regions, is a "ball and goal" full-contact sport played by two teams on a frozen ice surface. Instead of a ball as used in other hockey variants like field and roller hockey and in the similar sports of lacrosse and polo, the game is played with a thick disk of vulcanized rubber called a "puck". The puck is propelled along the ice (and sometimes through the air) by sticks with a flat blade carried by the players, who wear skates to move efficiently on the ice. The object of the game is to send the puck into the opponent's goal net to score a point, and the team who has scored the most points after three "periods" of play wins the game; one period is typically 15 minutes for amateur leagues, 20 for professional and international leagues. Players wear ice skates for propulsion, and protective pads to guard against hits from the puck or from opposing players' sticks or bodies.
Professional and high-level amateur leagues allow full-body contact to force players off the puck or otherwise interfere with their play. Offensive body blows, called "checks", are allowed in many common situations, but aggressive or restraining motions against a player made with arms, hands, legs, skate blades, or the stick, as well as "roughing" (an unnecessary check made against a player who is away from the active play), are typically illegal. In addition, many recreational leagues use "no-check" rules variations, where any offensive checking between players is penalized as "roughing" but other forms of contact (such as defending against an oncoming player by leaning into a hit) may still be allowed.
Each team typically has between 15 and 25 players available to use in a single game, depending on the league. Six of these players may be on the ice at any given time, usually five "skaters" (two "forwards" or "wingers", one "center" and two "defensemen") and one "goalie", although in certain situations the team may use six skaters and play with an "empty net". Skaters typically wear a helmet (with or without a visor or face cage), shoulder and chest pads, pants with pads over the tailbone and around the waist and upper leg, shin and knee pads with a hard outer shell in front, and skates with a thin, curved blade allowing high maneuverability while skating both forward and backward. The goalie, charged with preventing the puck from going in the net by any means necessary, is allowed to use special, more protective equipment, including wide, thick leg pads, a mask similar to modern baseball catcher's masks, a thick chest and arm protector, a wider stick, a "blocker" glove on the hand holding the stick and a "catch" glove on the non-stick hand. They also use special skates with a thicker, flatter blade that allows the goalie to more easily slide sideways off the skates into defensive positions.
Hockey is different from many other sports in that substitutions are allowed to be made "on the fly" without requiring a stoppage in play. Players may, at any point, simply skate to the bench and be replaced by the next available player for that position. Both players may be touching the ice at the same time, but as long as both are on the ice neither one may be "involved in play" and so players are encouraged to make changes quickly.
Two main variants of hockey are played on ice. The main variant, simply called ice hockey, is played in an ice skating rink; the exact dimensions vary depending on the league and level of competition, with the Winter Olympics and the international leagues using a wider sheet of ice (the same size as is used for "short track" speed skating) than for North American leagues including the NHL. The rink has a center line, two "blue lines" indicating each team's "defensive zone" (where certain rules such as off-sides apply while moving the puck into these zones), two goal lines and several "face-off circles" and "face-off dots" used to restart play after a stoppage, plus a wall around the playing surface with curved corners, helping to keep the puck in play and moving. Above the wall, plexiglass panels again help keep the puck in play while allowing spectators a close-in look at the action. Two nets, one on each goal line, are anchored to the ice with "breakaway" mounts (often a shallow spike) that prevent injury from a player colliding with the net.
The other variant is pond hockey, and is played on a natural body of water that has frozen over. The playing area is larger, but no walls are used (although there are boards to keep the puck in bounds), and the goal is much shorter (in fact only tall enough to allow the puck to pass through while sliding along the surface of the ice). There is usually not a designated goalie (and no special equipment is allowed), and the game is a "non-contact" sport (although players often still wear protective helmets and some pads).
Sled hockey is a variant of ice hockey used in competitions for the physically challenged; players sit on special sleds, and propel themselves with their hands, using shorter sticks to play the puck.
A related game often played casually is "broomball"; the game is played on a standard hockey rink, with ice that has been "roughed up" either by previous ice hockey play or by deliberate resurfacing. Players wear regular athletic shoes (no cleats) and any protective pads they may deem necessary to reduce injuries from falls, and use a broom or similar implement to push around an inflatable ball (the official ball is about the size of a canteloupe, but a larger soccer ball is common) and try to get it into their opponents' goal (the same nets as used for hockey).
Roller or inline hockey has much the same rules and equipment as ice hockey, but is played on a hard paved surface (often sealed concrete) by players wearing inline roller skates, using a hard hollow (non-inflatable) ball. It is often seen in the summer seasons and/or in warmer climates where naturally-occurring ice is rare, as a cheaper alternative to purpose-built ice rinks and the required refrigeration mechanisms.