Civilization II models the historical development of the human civilization. Each player picks one of a 20-odd historical civilizations, and plays as a significant leader from that civilization. The computer plays as competing civilizations, on a custom or randomly generated map. Each turn represents a certain number of years in human development.
Players begin in 4000BC, as a small settlement on a few tiles. The rest of the map remains unexplored, and the player does not have visibility beyond his own tiles and a few neighbouring tiles. Players slowly develop skills, make discoveries and invent technologies, while building the cities in their nation. They will encounter other civilizations which could be friendly, or hostile. Diplomatic relations can be established, technologies stolen, wars could be fought, and treaties signed.
The goal of the game is to either wipe out all other civilizations, or become the first space-faring civilization.
There are many approaches to play this game. One can be super aggressive, and have an all out war with every other nation encountered, until everyone is eradicated. Another approach would be to have a strong diplomatic ties, and use those ties to keep stealing their technological advances. The best approach would be a blend, by being agressive sometimes with some enemies, and have diplomatic relations with some others for mutual benefit.
The game makes you think, and gives you a great insight into the development of civilization and humanity as a whole, culturally, scientifically, politically, and technologically.
Based on the rules of this game, there is an open-source equivalent, called Freeciv. Definitely worth playing, strongly recommend.