As of the writting of this article, there are four PCIE specifications available, with increasingly greater bus speeds.
PCIE specification specifies a speed per lane. Version 1 specifies speeds of 2.5Gb/s (250MB/s), version 2 specifies speeds of 5Gb/s (625MB/s), version 3 specifies speeds of 8Gb/s (1GB/s), and version 4 specifies speeds of 16Gb/s (2 GB/s).
Typically, on a common consumer desktop motherboard, there are various combination of PCIE slots. For example, PCIE 1x, 2x, 4x, 8x, or 16x are commonly found on most motherboards.
Let's use a 16 lane example to compute what the maximum speed a PCIE card can communicate with the CPU. With version 1, the bus speed can achieve, in theory, 16 x 2.5Gb/s = 40Gb/s (5GB/s).
Faster bus speeds often reduces bottlenecks, for example, allowing more bandwidth between the PCIE graphics card and CPU.