PCI Express or PCIe is an expansion card standard that was designed to replace the older AGP, PCI and PCIx bus standards. The PCI-Express standard offers higher system bus throughput and has lower I/O pin count.
PCIe has serial links between devices called Lanes, unlike the parallel connections present in PCI buses. PCIe can be a x1, x4, x8 or x16 type where the number equals the number of lanes. ( D,E : ports and sockets of a motherboard )
The PCIe (common revision 2.1) has a bandwidth of 16 Gbps for a 16 lane link (x16).
PCIe slot can be used for placing expansion port card, modems or sound cards, etc. But, Graphics performance can benefit the most with the use of PCIe because they require the maximum bandwidth for transfers. So, the graphic cards (or GPU) are built for and placed in PCIe slots.