If you think of the processor or CPU as the brain of the computer then think of the bus as the nervous system.  A bus is a channel over which information flows between devices such as the video card, modem and hard drive. The bus speed drastically     effects    the    performance     of    the 

computer. The CPU must be synchronized with bus speed. The bus speed determines how well all of the internal components in a computer will work together. 

ISA Bus (Industry Standard Architecture)

The ISA bus is the original I/O bus used in personal computers for low-bandwidth adapters such as modems and sound cards. Older computers, typically those from before 1993, use ISA-based video cards as well. Modern computers do not have ISA bus slots, or they only have a limited number of ISA slots in order to maintain compatibility with legacy ISA I/O cards. ISA was originally only available as an 8-bit version, but eventually a 16-bit version became available. ISA I/O cards operated between 6 and 8 MHz, which was and is still sufficient for low-bandwidth applications. 

VESA Local Bus (Video Electronics Standards Association) 

The VESA Local Bus, also called VL-Bus or more commonly VLB was popular in computer systems as it provided more video bandwidth than the ISA bus. Introduced in 1992, VLB video became very popular during the 486 era. VLB cards can be easily identified by their longer slots. VLB video cards provided better performance than ISA cards, but was essentially replaced with PCI when Pentium-class computers were introduced.

PCI Local Bus (Peripheral Component Interconnect)

PCI was introduced in 1993, became popular as the Pentium came to dominate the market, and is now the standard of choice in most high-end systems. It offers 32-bit local bus performance and solves many of the problems associated with VLB, and introduces a host of new features including Plug and Play, burst mode-transfers and bus mastering. Bus mastering is an enhancement that allows a device to take over the system bus so that it can perform transfers to and from system memory directly. This improves performance on certain operations that uses the system memory. Bus mastering on modern PCs is currently supported only on the PCI bus. 

AGP (Accelerated Graphics Port)

AGP (64-bit) was designed for high-speed interfacing between the processor and the video card. Modern video systems involve a great deal of information that must be moved around, particularly between the video card, the processor and the system memory. The video system interface is the method by which the video coprocessor and video memory are connected to the rest of the computer. The video card requires more I/O bandwidth to the processor and memory than any other device in the system.  So much so, that video performance has traditionally been the driving factor for the creation of newer and faster system buses. Local buses were created to address the bottleneck in data transfer between the processor and video card that became acute when graphical operating systems became the standard. AGP addresses bottleneck problems by defining a new interface for video information that quadruples the theoretical bandwidth of current PCI buses. AGP is a port, and not a bus, because a bus can support multiple devices and AGP cannot. It is a point-to-point connection between the video card and the processor only.

CNR (Communication and Networking Riser)

The CNR specification, which was developed by Intel,  provides the PC Industry the opportunity to deliver a flexible and cost reduced method of implementing multichannel audio subsystems and broadband. The CNR Specification is an open industry specification and is supported by OEMs, IHV card manufacturers, silicon supplier and Microsoft. CNR has the capacity to minimize electrical noise interference, through physical separation of noise-sensitive elements from the motherboard's own communication.