Since the processor is considered the "brain" of the computer, the motherboard can be compared to the "nervous system." Nearly every component relies on the motherboard and is connected to it in some way. It is essentially a circuit board with paths or "traces" that transmit power and relay commands to the other components to tell them what action is needed. The motherboard links everything together, and helps the different components communicate.
When you execute any function on your computer, it requires a specific action. For each action an electronic impulse is sent to the motherboard. The board determines what the impulse is, what is needed, and which component must be implemented to complete the function. It then sends that component a signal. The component establishes what needs to happen and sends that information back to the motherboard. The motherboard then determines what action comes next and what other components are necessary to complete the task, sending them the appropriate command or impulse.
There are essentially 2 main types of motherboards, integrated and non-integrated. The term "integrated" refers to the computer peripherals. Integrated motherboards have things such as the sound and video controllers as well as ethernet embedded on the motherboard. Non-integrated motherboards only have the essentials for the motherboard, while things such as video and sound must be installed seperately. In the event that a user installs standalone peripherals onto an integrated motherboard, generally the standalone device takes precedence over the integrated controller. However, some motherboards have poor implementation of resolving this type of conflict, and the system will become unstable if there is both an integrated peripheral and standalone devices.
Another factor to consider when buying a motherboard is the form factor. The issue of form factor is covered more in-depth in the cases and power supplies section. Ultimately, you need to insure that the form factor on the motherboard and case match.