The Motherboard is the main system connection point for a computer system, to which the computer hardware and components are connected. It is relatively easy to connect new hardware to a motherboard, providing it is compatible and supported.
The first major element you see on a Motherboard is the CPU Socket. In the early days (Before the Intel 80386 Processor), CPU's were directly soldered onto the board, greatly limiting any upgrade potential.
With the Intel 80386, the sockets could be unlatched, and the CPU removed. This meant that Processors of the same Socket design could be used with that board, with minimal effort required for replacement. The square Socket design stayed until the Pentium II was developed, which had a rectangular slot design. This was duplicated with some AMD Processors, as well as with early Pentium III processors. Late Pentium III processors, and the newer Pentium 4 processors, use a traditional socket, instead of a slot.
The next major area for a motherboard are the expansion slots. The first type of expansion slot was known as ISA, and became the backbone for many years, until higher bus bandwidth was needed. EISA briefly appeared, and then, PCI was born. In the mid-1990's, AGP was conceived by Intel as a dedicated line for high-bandwidth Graphics cards, and enabled direct memory access without having to call the CPU. This later evolved into PCI Express when even higher bandwidth was desired.
Older motherboards had to be configured by means of jumpers (little pieces of plastic with metal inside used to make adjustments to the circuit), whereas newer motherboards can be configured increasingly through the BIOS or EFI BIOS. Such configurations must be made to tell the computer the specifications of the CPU, the boot order, and various other important settings.
See the Motherboard Category for a list of motherboards.