This option is what many people call the "tower" or "CPU". While the term CPU is innacurate, it has become common.
This is the box that all of the components are installed into. You have three choices: Full Tower, Small Tower, and Desktop.
A Full Tower is one that often stands at approximately 17 inches in height (height varies by brand), and has much room for expansion of system capabilities. It is often the easiest to work on.
A Small Tower is simply shorter in height than the Full Tower, often approximately 5 inches shorter. It can hold fewer hard drives and optical drives (CD-Rom, DVD-RW, etc.) than the Full Tower.
The Desktop is a very small case that typically is placed on top of the desk, and the monitor is placed on top of the Desktop. It can only hold one hard drive, and one optical drive, but it takes up very little space. They can also be more expensive than a tower case, but not always.