Are barebone computers just a motherboard and case?

"Typically, yes. They also include the processor heatsink/cooler that fits the chassis if the chassis requires a non-standard one.

You want to check the description and specifications to make sure what you get as there is no set standard."

Yes, you'd need to add a number of components. The linked configuration includes a case and motherboard, but the motherboard happens to have on-board audio, an on-board network adapter, and an on-board video adapter. This means you'd not only need to add hard drives, DVD drives, and memory, but you'd even need to add your own CPU. (Note the usage of the term "CPU supported.")