"Computer Main Board - An Introduction to the Motherboard"

All the critical subsystems, including the CPU, chipset, memory, disk drives, video and sound adapters, keyboard, mouse, and other peripheral components run directly on the Main board. Likewise, the interconnections among these components are laid into the Main board itself.
The motherboard performs two major functions:

• it provides physical support for the CPU, the memory modules, the expansion cards, and a long list of input and outputs, and
• it carries digital data through its printed circuit board to and from all of the other components that make up a computer.

If you are building a PC, choosing the motherboard is the most important decision you make and can be one of the most difficult. If you are buying a PC, the motherboard it uses determines its functionality and future upgradability.
Here is the graphical illustration of a computer main board:

Main Board Brand: Here is the most common motherboard brand that you can found:
Intel, Asus, Abit, Gigabyte, AOpen, MSI, PC Partner, AsRock, ECS, Acorp, Biosstar, etc
Each brand comes with lots of different models with slightly different board layouts, color and additional features.

What is included when you buy a computer main board?
1) The Computer Main Board - The physical board
2) Driver CD - Complement CD to install all the device drivers and utilities on the motherboard
3) Rear I/O Shield plate - to be install on the rear case
4) Main board user's manual - describe the details of the main board.
5) One 80-wire/40-pin ribbon cable or SATA data cable for HDD or CD/DVD drive
6) One ribbon cable for 3.5" floppy disk drives (optional)