Examples of different form factors: an ATX motherboard and power supply may take on several different external forms, such as a vertical tower (which sits on the floor) or a flat desktop or pizza box (which sits on the desk under the computer's monitor)). Full-size tower cases are usually larger in size than desktop cases, with more room for drive bays and expansion slots. Desktop cases and mini-tower cases designed for the reduced microATX form factor are popular in business environments as space is usually at a premium in those areas.

Although microATX and small form factors have become very popular for a variety of uses, as of 2007, the most popular form factor for desktop computers is ATX.  Apple Computer has created the Mac Mini computer, which is similar in size to a standard 5.25 inch CD-ROM drive.  Other companies like Shuttle Inc. and AOpen have popularized small cases, for which FlexATX is the most common motherboard size.

• Depending on the form factor and orientation, the motherboard is usually found screwed to the bottom or to the side of the case.
• Form factors such as ATX provide a back panel with cut-out holes to expose I/O ports provided by integrated peripherals, as well as expansion slots which may optionally expose additional ports provided by expansion cards.
• The power supply unit is often found at the top rear of the case; because of its considerable weight, it is usually attached with several screws.
• Most cases include drive bays on the front of the case; a typical ATX case includes four 5.25" and three 3.5" bays. In modern computers, the former are used mainly for optical drives, while the latter are used for hard drives, floppy drives, and card readers.
• Buttons and LEDs are usually found on the lower part of the front of the case; also in this area, some cases include additional I/O ports, or temperature or processor speed monitors.