CPU. You will need a Central Processing Unit (CPU).  There are only two brands of CPU: AMD and Intel.  The type of processor that you want will be a factor in determining what motherboard to purchase.  Specifically, you will want to know the brand, the series model, the socket type and the wattage required.

Memory.  Every computer requires memory.  Today, memory can be found in DDR2 and DDR3 versions.  The DDR3 is newer and faster and the version that most new motherboards are built to handle.  It is important to know what type of memory your motherboard is compatible with.

Graphics.  Some motherboards come with graphics chips already in them, others do not.  These will require a dedicated graphics card and the motherboard will come with a slot dedicated to handling graphics cards.

Motherboard.  The motherboard is the base upon which the above three components will be added.  Though it may come with an onboard graphics processor.  A motherboard comes in different sizes, known as form factors.  Different form factors work in different computer cases.

Hard Drive.  Today, there are two speeds associated with Hard Drive Disks.  3.0 Gb/s and 6.0 Gb/s.  Your motherboard will work with either, though it can only get the speed benefits of the 6.0 if it is designed for it.

Case. All computers and their various components require a case to house them safely.  Cases come in a variety of sizes and are geared towards housing particular types of motherboards.  The case specifications will include the form factors of motherboards that it can house.

A few items are still needed to round out a complete computer.  A power supply unit will be necessary.  This must produce enough power for your components.  The main area to watch for is the graphics cards.  They typically state a minimum wattage requirement.