DDR Interface Protocol
ddr is just a generic term meaning "double data rate", in which there is data associated with both edges of a reference clock, rather than just the rising edge or just the falling edge. It is normally associated with multi-bit buses, such as those used in memory chip interfaces. I have also encountered it in HD video interfaces.

PCIe does not use DDR. Instead, it uses a completely different signalling protocol known generically as lvds &mdash; low-voltage differential signalling. In this kind of interface, each pair of wires carries a "lane" of data in very high speed serial form (5 Gbps and up), with an embedded clock. For higher data rates, multiple lanes can be used in parallel, but they are decoded individually and the data is merged into a single stream afterward.