So-called "Barbarians" represent everything from the Huns to the Goths (all of em) to the Seljuk Turks to Tamerlane's invaders. They have played a pivotal role in history, sacking the Western Roman Empire, Byzantine Empire, many Germanic and Russian Kingdoms, the Indian Empires, China, etc.
Enough with the history lesson.
On Terra maps, you can see advanced Barbarian cities, with Maces around the time Rifles come into play. Many a times they have built The Colossus, The Great Lighthouse, etc before anyone else. Strange, but true. A
Couple hints on Barbarian Behavior:
1) Cities/Settler spawn rate increases when more tiles are covered by fog of war. This is true for all Barbarian units, but even more so for the former two.
2) When a Settler forms a City, 2 Barbarian units (depending on Era the barbs are in, I believe) spawn with the City to protect it.
3) Barbarians cannot build the Palace building; they have no capital and thus, their maintenance costs are either high or low (don't remember which, if someone could check the XML it would be nice...)
So yeah.