About the motherboard

A motherboard is the central or primary circuit board making up a complex electronic system, such as a modern computer. It is also known as a mainboard, baseboard, system board.
Most after-market motherboards produced today are designed for so-called IBM-compatible computers, which hold over 96% of the personal computer market today.
The basic purpose of the motherboard, like a backplane, is to provide the electrical and logical connections by which the other components of the system communicate.

Components and functions
The motherboard of a typical desktop consists of a large PCB. It holds electronic components and interconnects, as well as physical connectors (sockets, slots, and headers) into which other computer components may be inserted or attached.
Most motherboards include, at a minimum:
socket in which one or more microprocessors (CPUs) are installed
slots into which the system's main memory is installed (typically in the form of DIMM modules containing DRAM chips)
a chipset which forms an interface between the CPU's front-side bus, main memory, and peripheral buses
non-volatile memory chips (usually Flash ROM in modern motherboards) containing the system's firmware or BIOS
a clock generator which produces the system clock signal to synchronize the various components
slots for expansion cards (these interface to the system via the buses supported by the chipset)
power connectors and circuits, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.
Additionally, nearly all motherboards include logic and connectors to support commonly-used input devices, such as PS/2 connectors for a mouse and keyboard. Early personal computers such as the Apple II or IBM PC included only this minimal peripheral support on the motherboard. Additional peripherals such as disk controllers and serial ports were provided as expansion cards.
Given the high thermal design power of high-speed computer CPUs and components, modern motherboards nearly always include heatsinks and mounting points for fans to dissipate excess heat.
Integrated peripherals
Highly-integrated motherboards are thus especially popular in small form factor and budget computers.
For example the ECS RS485M-M, a typical modern budget motherboard for computers based on AMD processors, has on-board support for a very large range of peripherals:
disk controllers for a floppy disk drive, up to 2 PATA drives, and up to 4 SATA drives (including RAID 0/1 support)
integrated ATI Radeon graphics controller supporting 2D and 3D graphics, with VGA and TV output
integrated sound card supporting 6-channel audio and S/PDIF output
fast Ethernet network controller for 10/100 Mbit networking
USB 2.0 controller supporting up to 8 USB ports
temperature, voltage, and fan-speed sensors that allow software to monitor the health of computer components

History
Prior to the advent of the Apple II in 1977, a computer was usually built in a case or mainframe with components connected by a backplane consisting of a set of slots themselves connected with wires. The CPU, memory and I/O peripherals were housed on individual PCBs or cards which plugged into the backplane.
With the arrival of the microprocessor, it became more cost-effective to place the backplane connectors, processor and glue logic onto a single "mother" board, with video, memory and I/O functions on "child" cards — hence the terms "motherboard" and daughterboard. The Apple II computer featured a motherboard with 8 expansion slots.
During the late 1980s and 1990s, it became economical to move an increasing number of peripheral functions onto the motherboard (see above). In the late 1980s, motherboards began to include single ICs (called Super I/O chips) capable of supporting a set of low-speed peripherals: keyboard, mouse, floppy disk drive, serial ports, and parallel ports. As of the early 2000s, many motherboards support a full range of audio, video, storage, and networking functions without the need for any expansion cards at all; higher-end systems for 3D gaming and computer graphics typically retain only the graphics card as a separate component.
It can be argued that the motherboard industry was born by IBM in 1981 with the release their entry level 5150 Personal Computer (IBM PC) which was based on a motherboard. The motherboard provided an Intel 4.77MHz 8088 with 16K bytes of on-board memory, expandable to 640K through the use of plug-in memory boards, eight 8-bit ISA expansion connectors, cassette tape port and keyboard port. All other I/O such as the interface for 160K 5-1/4" floppy drives, serial and parallel ports were provided by plug-in boards. IBM approached Digital Research about using DR/DOS as an operating system but was rebuffed. IBM approached Microsoft and licensed PC-DOS. Microsoft released PC-DOS 1.1 in 1982 by retaining rights to the operating system allowing them to sell it to other manufacturers.
IBM published the schematics and I/O map allowing the birth of the clone motherboard industry.

Software Meets Hardware: the BIOS
A computer motherboard is a piece of hardware: it is the physical circuits and interconnecting wires that forms the backbone of a computer. It has logic circuits which can be manipulated and controlled by the operator, the software program, and input peripherals. But in order to begin operating from a power-off state, a motherboard must be bootstrapped (or simply, booted) by an initial set of software instructions. Without this vital software, the motherboard is rendered useless.
Most modern motherboard designs use a BIOS, stored in a EEPROM chip soldered to the motherboard, to bootstrap the motherboard. (Socketed BIOS chips are widely used, also.) By booting the motherboard, the memory, circuitry, and peripherals are tested and configured. This process is known as a Power On Self Test or POST. Errors during POST result in POST error codes, ranging from simple audible beeps from the speaker to complex diagnostic messages displayed on the video monitor.
The BIOS often requires configuration settings to be stored on the motherboard. Since configuration settings must be easily edited, these settings are often stored in non-volatile RAM (NVRAM) rather than in some sort of read-only memory (ROM). When a user makes configuration changes or alters the date and time of the computer, this small NVRAM circuit stores the data. Typically, a small, long-lasting battery (e.g. a lithium coin cell CR3032) is used to keep the NVRAM "refreshed" for many years. Therefore, a failing battery on a motherboard will produce the symptoms of a computer that cannot determine the correct date and time, nor remember what hardware configuration the user has selected. The BIOS itself is unaffected by the status of the battery.
When IBM first introduced the PC in the 1980s, imitations were quite common. (The physical parts which made up the motherboard were trivial to acquire.) However, the imitations were never successful until the IBM ROM BIOS was legally copied.To understand why copying the BIOS was an important step, consider that the BIOS contained vital instructions which interacted with peripherals. Without these software instructions in the BIOS, a PC would not function properly. (In most modern computer operating systems, the BIOS is bypassed for most hardware functions, but in the 1980s, the BIOS served many vital low-level functions.)
So when Compaq Computer Corp. Phoenix Technology soon matched their feat and began reselling BIOSes to other clone makers. It has been noted that Microsoft was more than happy to license the operating system (DOS), and IBM was more than happy to sue companies that violated the copyright of their BIOS. But by documenting and publicizing the reverse engineering of the BIOS, Compaq and Phoenix were legally competing with IBM using their own copyrighted BIOS.
Once the bootstrapping of the computer's peripherals are complete, the BIOS will normally pass control to another set of instructions stored on a bootable device.
Devices which are normally used to boot a computer:
floppy drive
network controller
CD-ROM drive
DVD-ROM drive
SCSI hard drive
IDE, EIDE, or SATA hard drive
External USB memory storage device
Any of the above devices can be stored with machine code instructions to load an operating system or a program.