The PCI Express bus (Peripheral Component Interconnect Express, written PCI-E or 3GIO for "Third Generation I/O"), is an interconnect bus that allows you to add expansion boards to a computer. The PCI Express bus was developed in July 2002. Contrary to the PCI bus, which runs in parallel interface, the PCI Express bus runs in serial interface, which allows it to reach a bandwidth that is much higher than that PCI bus.
The PCI Express bus comes in several versions (1X, 2X, 4X, 8X, 12X, 16X and 32X), which provide throughputs of between 250 Mb/s and 8 Gb/s, or close to 4 times the peak throughput of AGP 8X ports. Because its manufacturing cost is that similar to that of the AGP port, the PCI Express bus will progressively replace the former.
PCI Express connectors are not compatible with older PCI connectors. They vary in size and require less electricity. One of the interesting characteristics of the PCI Express bus is that it is hot pluggable, i.e. it can be plugged in or unplugged with out turning off or restarting the machine. PCI Express connectors can be recognised thanks to their small size and dark grey color.
The PCI Express standard is also intended to replace PC Card technology with "PCI Express Mini Card" connectors. What is more, contrary to PCI connectors which can only be used for to make internal connections, the PCI Express standard can be used to connect external peripherals by using cables. Despite that fact, it is not in competition with USB or FireWire ports.