From Civ World

The Barbarians are a rogue civilization that exists only for war. They do not appear as one of the civilizations foundable or joinable by players, and they cannot be visited or spied on. Instead, they appear only in battle. Barbarians have a unique set of military units that only they can field.

Any active civilization could be attacked at any time by a Barbarian army. When attacked, the civilization is forced into a battle exactly as if they had been invaded by another civilization. They can field whatever troops they have, and they can be assisted by armies from foreign civilizations.

Barbarians will not steal resources and Wonders if they win the battle. Instead, they will steal one population from every member of the defeated civilization.

If the civilization defeats the Barbarians, however, then the "most valuable player," or the player to contribute the most combined strength to the battle, will win a population bonus for his or her City. The bonus will be different depending on how many Citizens the prizewinning player currently has:

• If they have fewer than eight citizens, they will win one free citizen.
• If they have more than eight citizens, they will win 2000 food.

Note that while this is the same criteria by which players win Military Medals, players do not win Military Medals for defeating the Barbarians.