bueno aca esta lo que dice en wikipedia:
Cita: A barebone computer is a half-assembled platform that consists of a case with a power supply, a motherboard preinstalled into the system case, and a cooling system. Barebone computers are usually designed in a non-standard way and cannot be assembled out of widely available hardware parts. The purchaser of such a platform only has to equip it with a CPU, RAM, hard drive and additional input/output devices depending on his/her own needs.
Basicamente una computadora media armada. Es decir le puede faltar el disco duro, o la ram, o una tarjeta de video o etc etc...