An Acronym for Peripheral Component Interconnect.
PCI-X is available in 64bit and 66, 100, 133, 2662? and 533Mhz2? variants. Mostly only used in servers for various peripheral cards that require high bandwidth like RAID, Gigabit Ethernet and Fibre Channel HBA cards.
PCI-Express does away with the traditional Bus architecture of PCI and replaces it with a packet-based point-to-point architecture. Each slot is available as a number of full-duplex lanes from 1 to 32 combined to form a link that's connected to a switch. Each lane is capable of transmitting 2.5Gbps in each direction. 16x slots, at 40Gbps, are being used to replace AGP slots for graphics adapters. PCI-Express is backwards compatible with PCI from a software point of view, so an OperatingSystem does not need any modification to support PCI-Express hardware.