Essentially, PCI is considered a local computer bus where hardware devices in computers get attached. Standing for Peripheral Component Interconnect, PCI supports the processor bus but is independent of any native processor bus when it is used in the standardized format.
The devices which are attached can be integrated circuits that are fitted to the motherboard or to an expansion card that goes into a slot. The devices that are connected to the PCI bus are usually a bus master that is connected directly to its own bus with an assigned address space. The PCI Local Bus was first created for IBM PCs where it replaced the slower ISA slots along with a fast VESA Local Bus slot in the configuration.
From there, it expanded into other types of computers and used in network and sound cards along with modems, disk controllers, TV tuner cards, and even extra USB or serial ports. For years, the PCI video cards replaced the VESA and ISA versions until the bandwidth requirements grew beyond its capabilities and it had to be replaced. After the PCI video cards, the AGP replaced them before the PCI Express was introduced.
Work began on PCI in 1990 and it was not until 1994 that it managed to really penetrate the market. They continued to be used in computers at a very large rate until the mid-2000s when the Conventional PCI versions started to be replaced. Today, many of the new motherboards do not have slots for Conventional PCI.