The motherboard is the central part of your system it contains different circuit and components either directly on the board or by additional components which connect directly to it. The motherboard is also commonly referred to as the main board.
The motherboards contain many connectors; slots and include a pin less or with pin socket for the processor to be installed along with memory slots, a number of expansion slots called as PCI slots, and connectors to IDE/SATA devices and smaller connectors for USB, serial and printer as well as network LAN connectors.
The motherboard plays a very important role in the following aspects of your computer system:
As everything gets connected to the motherboard and it is designed in such a way that lay out dictates how the entire computer is going to be organized.
The motherboard has it own BIOS program and a chipset which help in controlling the data to transfer from one computer to many other computer systems.
Communication System: Approximately all communication among the PC and its peripherals, other PCs, and you, the user, goes through the motherboard.
The socket in your motherboard depicts which type of processor can support your motherboard whether pinless or processor having pins.
There are many more other connector which the motherboard support like PCI slots namely called as Expansion slots or the AGP or PCI Express slots for graphic card and even have some small ports like USB and LAN connectors and there are many more connectors available as buy on high-end motherboard of the next generation.
The System Performance depend on your motherboard as it plays a major factor in your performance as it dictates which type of processors, memory, system buses, and hard disk interface speed your system by changing the setting in the BIOS or Overclocking the GHz of the processor as it can be done in some of the latest motherboard . Often if you are upgrading after a number of years you will need to replace the motherboard memory and also the processor.
As the motherboard are designed according to the latest processor which are compatible with the hardware and also with the limitations of the circuitry built-in to the board itself will not allow them to run. As the next generation motherboard comes upgraded with HDMI support, Crossfire and etc.
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The block of connectors on the motherboard which include the Ethernet, USB, serial, com and mouse/keyboard ports is often referred to as the I/O Panel, the often silver surround which sits between the panel and the outer edge of your system case is called the I/O shield.

Expansion slots
Usually PCI to allow for other devices to be installed, AGP cards are used solely for video cards. Newer versions including PCI-Express functions still remain the same.
Processor socket

Here your processor (CPU) is connected to the motherboard. Usually they have a socket design with a lever on one edge once lifted you can insert the processor. Normally a small notch or marked corner indicates the only way that the processor can fit the socket. Once secured the heat sink and thermal paste is installed over the processor and secured into place.
Memory slots

Also known as memory banks as each slot on the motherboard is termed a bank of memory. You will need to check that you are using the right type of memory for you board. They will only fit into the slots one way by noting the notch in the middle of the module and the number of pins on each side. Once the clips at either end have been opened the module can then be pressed firmly into the slot and the clips secured to hold the module in place.
IDE Connectors

These connect to your IDE devices of either hard drives or CD/DVD Drives or a combination of both. A smaller IDE connector is also used for Floppy disk drives.
Pin outs
Every switch on the front panel of your case including the power switch, reset, power led, hdd led, and any USB ports all need to be connected to the pin outs on the motherboard to function.
Others
Other connectors on your motherboard include additional pins to connect fans to. If you have built-in sound you will also see AUX and CD-in which connect to the DVD drive to allow for playback directly from the drive.