Mainboard

The mainboard must have a socket for at least one CPU chip, and sockets for typically 2 or 4 memory "sticks". The back panel will expose plugs for keyboard, mouse, USB, Ethernet, and maybe External SATA disks. Some mainboards have integrated video. It will then have slots and connectors for the things you add it: SATA connectors for disk and PCI and PCI-e slots for adapter cards.

Things to look for in a mainboard:

• The mainboard has to match the CPU. A mainstream Intel desktop mainboard has a Socket LGA775, new Intel Core i7 processors only plug into $300 mainboards with a Socket LGA 1366, while an AMD mainboard has an AM or AM+ socket.  Each board vendor has a Web page that lists CPUs supported by a board. Make sure they match. For more information ...
• Mainboards come in standard (ATX) and small (MATX) sizes with a few other (jumbo, tiny) sizes used only for specialty purposes. The small MATX board will only have two memory slots and room for four adapter cards, but it will fit in media center cases designed to look like stereo equipment. MATX boards almost always have integrated video but you can upgrade to a separate video card if you need it. These days only consider an MATX board with digital (DVI or HDMI) video output capability. If you get an ATX board you have a lot of card slots, so decide how many old PCI slots you need for old cards and how big the new PCI-e slots have to be for new cards.
• A mainboard with "all solid state capacitors" is better quality and gives you one less thing to worry about.
• A high end mainboard (over $200) typically has slots for two or more video cards (SLI or Crossfire configurations). Unless you play video games, the extra board cost and slots are not useful. However, if you plan to run Quad core AMD Phenom processors you may need a high end board just to get enough power to drive the CPU chip. 
• Today disks and DVD drives all come with the better, simpler Serial ATA connector. If all your devices are SATA, then make sure the mainboard has enough SATA connectors to handle your immediate needs and leave room for expansion. Four is barely enough, while five or six is better. If you have an old parallel ATA device, then make sure your mainboard has at least one parallel ATA socket into which you can plug one of the old flat cables.
• Look at the external connectors on the back. Some board vendors waste space by putting the ancient big parallel printer port connector even though all printers made in the last decade use USB. The old round "PS/2" mouse and keyboard connectors are still useful, although most new input devices use USB. It is a plus if the mainboard has External SATA (eSATA) connectors for external disks, and if you are going to connect your computer to a theater sound system you should look for a digital audio out connector. Any mainboard with built-in video should have at least a DVI or perhaps an HDMI connector.
• Gigabit Ethernet is useful, particularly when you move large files from machine to machine in a home network.

For all the different things that a mainboard does, it is astonishing that perfectly reasonable boards sell for $50. An enthusiast who wants to run two, three, or four video cards for gaming can spend $300, but most of the extra cost supports cooling the chips when you run the board at higher than the standard clock speed.

A mainboard has lots of different functions and connections. While it is possible for something critical to fail, most mainboard problems involve specific functions. A USB port doesn't work, or the built-in audio doesn't work, or the round PS2 connector for the mouse fails. These functions can be replaced with adapter cards or external USB devices. At some point the patches and limitations become annoying enough to buy a new board.