PCI-SIG (the Peripheral Component Interconnect Special Interest Group), defines PCI Express as "...an open specification designed from the start to address the wide range of current and future system interconnect requirements of multiple market segments in the computing and communications industries. The PCI Express Architecture defines a flexible, scalable, high-speed, serial, point-to-point, hot pluggable/hot swappable interconnect that is software-compatible with PCI."
That's a pretty dense specification summary, so we'll go over it at a bit more length. Firstly, PCI Express is an open specification, meaning anyone can implement PCI Express. Although Intel will be the first manufacturer to debut the technology, representatives at NVIDIA have commented that the firm will also support PCI Express in an upcoming version of the nForce3 chipset once Intel unveils the Grantsdale and Alderwood chipsets. Moreover, SiS already announced its own 965 South Bridge, which incorporates two PCI Express 1x connectors, in addition to eight-channel audio, four-port Serial ATA, and integrated Gigabit Ethernet.