Peripheral Component Interconnect Slot.
An expansion slot on many motherborads used for inserting various cards to
upgrade your hardware and/or support for features not inluded on the mobo itself.
PCI Sound Card - Adds the capability of producing audio with a computer :).
PCI Graphics Card - If you still use these types of cards your games probably look as crusty as the pcb of your graphics card.
PCI USB/FireWire Card - Adds the ability to use USB and FireWire capable devices if your mobo didn't already support it.