The motherboard or more colloquially "mobo," also known as the mainboard, connects to the power supply via a large connector. The mainboard is the biggest component in size in your PC, and the most important as well. It connects everything — expansion cards (like graphics cards for example) plug into it, the chip that controls your hard drive (called a controller) is on your mainboard, and much more. The type and composition of motherboard in your system determines its upgradability.

Wired to the main board are several things, including on-board Ethernet, video, modem (this is becoming optional), audio, memory sockets, processor socket, various connectors for USB, expansion cards, chipset, subsystems, and of course lots of circuits that link everything to everything else. In most cases, the on board features such as video and audio can be used just as they are when you sign on. These are limited to basic operation only, which is why you can't typically play serious video games on a system with on-board video; for graphics-intensive games you will need a graphics card or even two. In recent years PC makers have partly solved this issue by allowing the video subsystem to use shared memory with the main memory bank used by the PC for all its operations, allowing some increased functionality for video. You won't usually find serious audio hardware of any kind on most PCs, though mainboard manufacturers are typically including an HD audio chip, which can be useful with future upgrades. You can upgrade your video with a crazy insane graphics card, and add HD audio or surround sound to your PC via the expansion slots. Upgrade components are generally not all that pricey these days, but once again, what your mainboard will handle can be an issue if it is older or cheaper.