usually only with the components needed to start a computer, mobo, psu, heatsink and hard drive/floppy drive/video card, RAM sometimes just a motherboard inside, varies on the seller really what they define as barebones.
No. Most do not include hard drives, CPU and memory. A lot are just case, motherboard with lots of integrated componants (sound, NIC, video, USB...), and floppy. CD-ROMs are sometimes included, as are keyboard, mice, and speakers.