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.

The first thing you should note about your motherboard is the CPU socket type.  CPU are made to work with a particular type of socket.  If your board does not use that socket, no amount of force will ever make the CPU and the motherboard work together and your new pc will not work.

The types of memory supported is also important.  DDR2 and DDR3 are the two types found today, with DDR2 the older and slower form of the two types.  The motherboard will list what types it is compatible with.  It will also list the data rates of the memory that it is compatible with.

Graphics are a necessary component and some motherboards will come with on-board graphics cards while others will not.  Additionally, it is important to note how many and what type of graphics cards the motherboard will support.  The on board graphics will work well for simple applications and computing, however games will benefit a great deal by the addition of a graphics card.  It is important to figure out what type of graphics card you will need and then make sure that the motherboard will support it.

Lastly, motherboards support certain types of USB connections and SATA connections.  The new USB form is 3.0 and the new SATA interfaces (for connecting to HDDs) is 6.0Gb/s.  If you are using an HDD that uses 6.0Gb/s, then you will want a motherboard that can use it at that speed, otherwise the motherboard will be limited to SATA 3.0Gb/s.

In summation, it is very important that you first understand what you need when looking at motherboards.  If you are looking for a budget pc, then the older models will work fine and there are still plenty of older components on the market.  If you are looking for something with upgradeable potential, then there are lots of good choices too.