PCI-X is an obsolete expansion card specification developed by IBM back in the 90s for use in server environments. Was this what you wanted to know about or is it PCI-E?
PCI-E (PCIe) is the current expansion card specification used in modern PCs. There are different slot sizes such as a PCIe x1 slot size (a small slot for small add-in boards) or PCIe x4 slot size (a bigger slot for bigger add-in boards) and PCIe x16 slot size (a large slot used mostly for graphics cards).
Also a PCIe x16 slot can be configured to run under different modes depending on what a motherboard manufacturer has decided to configure. For instance, depending on the mode, a PCIe x16 slot can run with 16 PCIe "lanes", 8 PCIe "lanes", 4 PCIe "lanes", etc. More PCIe "lanes" means more bandwidth (speed) for the device.
Also, there are three main specifications of PCIe "lanes": PCIe version 1, PCIe version 2 and PCIe version 3.
One lane of PCIe version 2 is twice as fast as one lane of PCIe version 1.
One lane of PCIe version 3 is twice as fast as one lane of PCIe version 2.