I always thought of the motherboard as a convenient way of holding all the electrical components together. It has the various chips, some resistors, some plug mounts, and a bunch of electrical connections (i.e., wires) printed on. In the old days, you could just wire all the components together without a printed circuit board and have all the buses and what-not. The board part of the motherboard may not technically be required, but it makes life a lot easier to have it.
The communication buses and what-not are at a higher conceptual level than the wires that connect the components together. Sure, the buses have requirements that the wires offer a certain range of electrical resistance and that the signal propagate within some time frame (i.e., that the wires have some maximum and minimum length).
But the signal bus is not the wire. The bus is more than the wires.
All of that written to shift the frame of reference before one goes and follows @ldigas suggestion of doing some research into this really broad and open ended question.
One additional note: language being what it is, the term motherboard generally does refer to the printed circuit board, the CPU socket (and sometimes includes the CPU), the chipsets that support the CPU (i.e, North bridge, South bridge), the RAM sockets, the chips that hold the BIOS, the PCI sockets ... i.e., all the stuff glued and soldered to the circuit board. But if you are trying to understand how those little parts work together to give you a computer, I find it useful to remember that the little parts still work even if not wired together on an actual motherboard.