The motherboard, or mainboard, is called that because all of the other components plug into it, and it is used as a hub of sorts for all of the components to communicate with each other. When buying a motherboard, the most important things to know is what CPU it is compatible with, what kind of memory, or RAM, it is compatible with, and the speed of the frontside bus.

CPU Compatibility:

The two main types of Motherboards follow along with the two main brands of CPUs, being AMD or Intel. An AMD CPU will NOT work in a motherboard configured for Intel CPUs and vice versa. Also, most motherboards have a range of speeds that they are compatible with (example) like 500 Mhz to 1.0 Ghz. Any higher would require a different motherboard.

Speed is not the only factor though, there are a lot of different styles of CPU, and each motherboard should tell you exactly what type of CPU it accepts. These are the reasons why you typically replace the CPU and the Motherboard at the same time, because they are so dependant on each other.

Dont worry if this is all going over your head, they sell bundles for just this occasion called Barebone Computers, which is usually a case, motherboard and CPU combo so you cant go wrong with these.

RAM Compatibility:

There are various types of RAM these days, which are described here, but to note them quickly, they are SDRAM, DDR and RDRAM. They all serve the same basic purpose, just in a slightly different way and speed. It is important to know what type of RAM your motherboard takes and the maximum amount you can have in it at one time, which is usually noted in the description of the motherboard.

Video Card Compatibility:

Video cards are not all created equally, there are several different types of video cards now, especially on the high end. The most common being AGP, and PCI Express. Though there are also various types within these groups. PCI Express is still new and doesn't have many variations yet, but AGP has been around and has several different variations, like AGP 4x, and AGP 8x, etc.

Most of the newer versions are backwards compatible, meaning that they will accept older versions, but it does not work the other way around, so you usually can not put an AGP 8x card into a motherboard that is labeled as only accepting up to AGP 4x.

Hard Drive Compatibility:

There are couple different types of hard drive interfaces today, being IDE, SATA, and SCSI. These interfaces are not cross compatible, so you need to know what kind of hard drives you have, or intend to purchase, and make sure that the motherboard you purchase supports the interface for that hard drive.

Front Side Bus (FSB):

Front Side Bus is important to know because if your RAM and CPU have high speeds and your motherboard does not, communication speed is limited and creates a bottle neck. While it is not necessary to get all of them exact, it is good to try to get them as close as possible.

Some other things to note about choosing motherboards are what features are built into them. These days it is not uncommon to see Audio, Lan and Video built into your motherboard. If you dont need amazing audio sound or an expensive video card, its usually sufficient and MUCH cheaper than buying all of these separately.