DDR means "double data rate" and basically runs "twice" as fast as SDRAM since it reads on both edges of the clock cycle.
SDRAM is slower than DDR, and DDR is slower than rambus (this is debatable though!)
Rambus is used in all the new pentium systems. All the older PIII, PII's etc, still use SDRAM
DDR is only currently used in AMD athlon systems.