Unless only one civilization remains, the winner is the player with the highest score. Points are awarded for the size of a civilization, its wealth, and cultural and scientific advances.
The game was inspired by the commercial computer game Civilization (and sequels), which are in turn based on the board game of the same name. Freeciv is quite configurable, so it can be played in Civilization I, Civilization II or Freeciv mode, or you can write your own mode.
Freeciv is free software under the GNU General Public License.