First came PCI, then PCI-X, then PCI Express. PCI Express can be abbreviated as PCIe or, less commonly and more confusingly, PCX. Unlike earlier PCI standards, PCI Express does not use a parallel bus structure, but instead is a network of serial connections controlled by a hub on the computer's motherboard. This enables PCI Express cards to run significantly faster than previous PCI cards.
Because the PCI Express interface is a serial connection, it does not have a speed measured in Megahertz, like PCI or PCI-X. Instead, its performance is measured in data throughput speeds, which are several times faster than PCI-X. Furthermore, PCI Express is available in x1, x4, x8, and x16 implementations, which increases the bandwidth by the corresponding amount. However, larger implementations require longer PCI Express slots. For example, a x4 slot is larger than a x1 slot and a x16 slot is larger than a x8 slot. A PCI Express card can be inserted in any slot that is large enough for it. For example, a x8 card could be inserted into a x16 slot, but a not a x1 or x4 slot.
Since PCI Express connections can support such fast data transfer rates, they can be used to connect high-speed devices such as Gigabit Ethernet cards and high-end video cards. For this reason, PCI Express is expected to replace both PCI and AGP connections. Fortunately, PCI Express was designed to be backwards compatible with both PCI hardware and software. However, to use a PCI Express card, your computer must have at least one available PCI Express slot.