Classical mahjong rules have been changed many times. Nowadays there are many different variations of them. Most popular of mahjong rules are Chinese version, United Kingdom version and United States of America version. In USA version of rules many special hands were added which made this version more strategic than other ones. Japanese mahjong rules are more faster - the main goal is to leave first.
Computer mahjong games add even more twists to mahjong rules. There are many different tile sets, extra bonuses or special tiles. Often classical mahjong rules are combined with action or arcade gameplay. But almost every game has a game mode with classical mahjong rule. So you can always return to the basics of mahjong.