To play Mahjong, you need four players, the requisite 136 or 144 tiles, two dice and four racks to hold the tiles, if available. The winner is ultimately she who arranges her tiles into three melds (sets) and one pair without any discard tile.
Each player rolls the dice, and the highest roller deals. The dealer then mixes all tiles around in the middle of the table, face down, so that no one sees their values. She then deals each player 13 tiles, with the person to her right going first. There are three types of melds; a pung is three identical tiles, a kong is four identical tiles and a chow is three tiles of the same suit aligned in proper sequence.
Each player draws a tile from the center of the table, then discards one into the discard pile. In this sense, Mahjong is quite like Gin Rummy, though with 13 tiles maintained at all times. Other players can then pick up the discarded tile to form a meld, though tiles used to form chow melds must be taken only from tiles discarded by the player to one's left. Melds formed from discarded tiles must be shown to the table, whereas melds solely formed from tiles from the draw pile needn't be. Scoring systems vary, often with only the winning hand being counted.