This is an expansion pack for the game Civilization 3.
The expansion pack improves several aspects of the game, including more civilizations, a multiplayer mode, new game modes, more wonders and new units.
8 new civilizations were added to the game, the Arabs, Carthaginians, Celts, Korea, Mongolia, Ottoman Empire, Spain and Scandinavia.
The main feature in the expansion pack, multiplayer, was originally very buggy and problematic, players often being constantly disconnected and getting out of sync issues. Several patches were released for the game, and eventually almost all the issues were worked out. LAN play was almost perfect, making disconnects and out of sync issues very rare. Internet play was slightly less stable, but was within acceptable parameters.
Really if you have some friends that are into this kind of game, then you need to get this expansion pack, or alternatively get the Civilization 2 Multiplayer Gold Edition, or just get Civilization 4, which contains multiplayer modes with the original game. AI is also supported in multiplayer modes, so even if there are only 2 of you, you can have some good fun together. The game supports up to 8 players, unlike singleplayer modes which I believe support 12 civilizations.
This expansion pack is included in Civilization III: Gold, Civilization III: Conquests or on its own. However you will still need Civilization III to play the game.