See the Motherboard Category for a list of motherboards.
The Motherboard is the main system connection point for a PC, to which the computer hardware and componentry is connected. It is relatively easy to connect new hardware to a motherboard, providing it is compatible and supported, thus upgrading the computer.
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 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. The square Socket design stayed until the Pentium II came to the scene, 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 socket, not a slot.
The next major area is 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 sought. 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 Graphichs Cards, and enables direct memory access without having to call the CPU.
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. Such configurations must be made to tell the computer the specifications of the CPU, the boot order, and various other important settings.