PCI

From Mac Guides

The Peripheral Component Interconnect standard (almost always shortened to PCI) specifies a computer bus for attaching computer peripheral devices to a motherboard. PCI cards can be installed on any appropriate computer running any operating system, assuming software drivers are available. PCI cards include USB and FireWire add-in cards, ATA cards, and even Graphics Cards and TV tuner hardware.

The PowerMac G5 introduced PCI-X cards to the Macintosh world, and recently Apple's line has begun making the transition to the PCIe (PCI Express) architecture. PCI-X and PCIe cards are not compatible with PCI 2.1/2.2 cards, though both PCI-X and PCIe slots are backwards compatible in certain situations.