From LQWiki
A motherboard or mainboard is the most significant component in a modern personal computer. It is a printed circuit board (PCB) which contains most of the semiconductor components and all of the electrical connectors necessary for other essential components and devices (graphics card, keyboard, etc.) can be attached to make a working computer.
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Modular Construction
The motherboard facilitates modular computer construction. Rather than replace everything to make an improvement, it is possible, e.g., to only replace the graphics card or insert more software memory.
However, as certain components become more widespread, and the cost of manufactoring decreases, some subsystems are placed directly on the motherboard. For example, in 2004, most motherboards now have Ethernet/LAN capabilities included and there is no longer any need to add an extra card into one of the connectors on the motherboard to provide networking.
But in another trend, as certain subsystems develop faster than the rest of the hardware, these systems are spun-off into seperate cards. For example, before 3D acceleration became popular, video was intergrated into the motherboard. With the rise of 3D acceleration, GPUs were spun off into seperate expansion cards.
These two trends combine to form the wheel of reincarnation.
Alternate Terminology
In the early days of the motherboard (mid to late 1980s) the term daughterboard was used to describe PCBs that would have been inserted into the motherboard (c.f "child" processes in software and systems design). The term never really caught on.
It is possible that the use of mainboard as a synonym reflects increasing attention to gender neutral language as well as providing greater indication of the hierarchy of importance of computer components.
Among computer geeks, the term "motherboard" is often shortened to "mobo".
History
Originally computers were all hardwired. Everything needed was built as a single unit with little or no opportunity for repair or extension. There were no modular components. As the industry developed, and semiconductor technology developed, it became possible (technically and economically) to create modular components.
For this modularisation to continue, and with an early nod to interoperability, "backplanes" were developed. Backplanes, (sitting at the "back" of the cabinet) were PCBs replicating hardwiring. Originally a backplane was no more than many straight copper tracks connecting pins on connectors in parallel. Apart from special provision for power supplies, one backplane was much like another. What differentiated them was a specification that defined which connector pins did what. For example, defining the address bus, the data bus and control signal lines. Standard size PCBs, containing, e.g., the central precessing unit, a disc controller or memory were plugged into any position on the backplane to create a computer.
When backplanes were being developed, integrated circuits were not particularly complicated. In the 1980s it was still considered viable to build a central processing unit from discrete logic components and sub units (e.g., AMD 29xx technology) Double sided PCBs (copper on both sides) were "cutting edge". In the early 1990s multi-layer PCBs multi-copper insulator sandwiches, were the stuff of research and development projects.
As the associated enabling technology developed (PCB design and manufacture, component complexity) backplanes became more complex and started include some of the technology we now recognise on a motherboard
In 2004, the motherboard, with seven or eight layers of copper, is an example of manufacturing precision and is critical (topologically) to enabling a modern complex central processing unit, Pentium, Athlon, etc., with hundreds of pins (connectors)to be able to be connected to the embedded address, data and other buses to other complex components (north bridge, south bridge etc.) resulting in the modern personal computer.
The backplane lives on in the rows of PCI, AGP, and other connectors that exist on a motherboard.
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