To completely explain barebones systems would take a while, but I'll try the best I can to do it quickly.
Basically, barebone systems provide the basic essentials to build your own computer. They have a tower and motherboard and powersupply always. Many contain a CPU chip that fits into that specific motherboard that is in the tower provided. Some even contain small amounts of ram (256-512MB) to start off. None include a GPU, sound card, hard drives, IDE or SATA cables, etc. Barebones are the "barebones" people need to start building a computer, they are like a kick start to get your project accelrated and going quickly without having to have to worry about MOBO sizes, pin confgurations, Powersupply being adequate, etc. Hope this answers your question. That is the best I can do without typing one whole page on them.