More specifically, see civilizations/civ4civilizationinfos.xml and search for
<Type>CIVILIZATION_BARBARIAN
There is a <Buildings> tag under that which lists a building class type and then the keyword NONE. This is oddly named, but it prevents the barbarian civilization from constructing that building. Similarly, the <Units> tag lists the units the barbarian may not construct. So if you want barbarians to construct, say, musketmen, then remove the <Unit> section (4 lines) for UNITCLASS_MUSKETMAN.
I don't know how the game engine handles technology requirements for barbarians. It would be annoying if you removed the <Unit> section for, say, tanks, and the barbarians started constructing tanks during the ancient era. We know that barbs build catapults, but I would be pleasantly surprised if the game actually tracks development of the construction tech for barbarians.