A traditional
Chinese game that is popular in a number of east asian countries. The game is based on a set of 144 tiles and requires four
players. The tiles are composed of sets of four identical tiles (except for two sets that are unique and have special meanings).
Actually, there are two very different games played with these tiles, the simpler of which is played by only one person and consists of removing all the tiles from a tower-like structure in pairs of identical tiles at a time. This game is not really Mah Jongg, but often mistaken for it by Westerners because it is much more widespread (at least as a computer game). The reason for this is probably that it is very easy to learn, yet suffciently difficult to be entertaining. It's also often called Shanghai.
Real Mah Jongg has very complex rules that take some time to learn and (of course) much longer to master. Basically, each player keeps a (hidden) hand of 13 tiles and tries to build one of a number of combinations called "Mah Jongg" by discarding useless tiles and getting replacements from the "Wall", the collection of unused tiles. However, discarded tiles can be picked up by other players to finish sub-combinations which they then have to reveal. This way, players have partial knowledge about the other players' hands and have to be careful what tiles they discard, lest they give the opponent the last tile he needs to win.
Once a player has completed a Mah Jongg, the round ends and all players' hands are scored. After a certain number of rounds, the total score determines the overall winner of the game.
This overview of the rules is very simplified and just intended to give a general idea of how the game works. There are many special cases and my description may even contain some outright errors; if you find one, please msg me with corrections.