The individual components of the system board are interconnected by
multiple connection systems known as buses. The system bus is divided into
three logical functions; the address bus, the data bus and the control bus.
The address bus specifies the memory locations (addresses) for the data
transfers. The data bus holds the data transfered. The data bus is bidirectional;
it allows data to be read into the CPU and written from
the CPU. The control bus contains various lines used to
route timing and control signals throughout the system.
Many flavours of buses exist, for example ISA and
PCI buses are popular ways
of connecting peripherals to the system.
Details on the various bus types can be found here