PCI stands for Peripheral Component Interconnect. Most Pentium PCs come with three of these and up to five 'ordinary' 16-bit slots. The best graphics controllers need PCI expansion slots.
Some 486 PCs are also fitted with PCI expansion slots, though others may have a mixture of ordinary 16-bit slots and an alternative to PCI which is called VL-Bus. The VL-Bus standard is rapidly being overtaken by PCI and plug-in cards for the two systems are not interchangeable, so go for PCI if you have a choice.