The heart of any motherboard is the various buses that carry signals between the components. A bus is a common pathway across which data can travel within a computer. This pathway is used for communication and can be established between two or more computer elements.
The PC has a hierarchy of different buses. Most modern PCs have at least three buses; some have four or more. They are hierarchical because each slower bus is connected to the faster one above it. Each device in the system is connected to one of the buses, and some devices (primarily the chipset) act as bridges between the various buses.

Bus Hierarchy:
1) Processor Bus
2) Cache Bus
3) Memory Bus
4) Local I/O Bus
5) Standard I/O Bus
6) AGP bus
7) PCI bus
8) ISA bus
if you want to know about the bus hierarchy and more details, use this link
http://www.pcguide.com/ref/mbsys/buses/func.htm
http://geekscomputer.blogspot.com/2008/07/system-bus-types-functions-and-features.html