PCI express (PCI-E or PCIe) is an improved version of PCI that doubles and expands on data transfer rates. PCI is the standard connection interface for connecting the PC motherboard to computer peripherals that extend the computer's capabilities.
PCI-E utilizes a serial interface as opposed to the old parallel interface the original PCI uses. PCIe also uses specialized buses for every device connected to it while PCI uses a shared bus. There is a large speed difference between the two. A standard PCI slot runs at 133 Mb/s while 16 slot PCI-E can run at 16 Gb/s. PCI express was developed to facilitate the high transfer speeds of high-speed interconnects.