An expansion bus is an assortment of wires that allows for computer expansion with the use of an expansion board, a printed circuit board inserted into an expansion slot on the motherboard or backplane that provides additional features to a computer system.
An expansion bus provides an input/output pathway for transferring information between internal hardware, such as RAM or the CPU, and expansion devices such as a graphics card or sound card.
Techopedia explains Expansion Bus
There are several different types of expansion buses with diverse features, such as:
Industry Standard Architecture (ISA)
Accelerated Graphics Port (AGP)
Peripheral Component Interconnect (PCI)
PCI Express (PCI-X)
One of the early types of expansion buses was the ISA, which was used with IBM-compatible computers. It supported a network card, video card or additional serial ports. However, the ISA bus had a tendency for bottlenecks and was replaced by the PCI bus. The PCI has faster standards, with a 33 or 66 MHz bus speed and support of 32- and 64-bit data paths. The PCI Express bus, which replaced the PCI, is even faster.
The PCI interface is used in many modern computer interfaces and conforms to the local bus standard developed by Intel Corporation. Although the PCI Express (and the PCI-Extended) has a faster interface, the PCI is more common and can even be found on some versions of Macintosh computers. Prior to the PCI interface, some PC architectures, such as the NuBus and Macintosh, used their own buses.