Instead of explaining, I found a good explanation of why DDR is what it is in terms of PC RAM:

With standard SDRAM, data is transferred from the memory to the processor when the clock signal bounces from LOW to HIGH. With DDR, data is transferred not only when the clock signal goes from LOW to HIGH, but also when the clock signal goes from HIGH to LOW. Voila! Twice as much data on each tick of the clock!

I've heard of PC1600 and PC2100. What does this mean, and what is the difference?

Well, it's kind of like PC100 and PC133. Since it is double datarate, you might expect that DDR would be PC200 and PC266. In fact, the RAM chips themselves are sold as PC200 and PC266. HOWEVER, us module guys did not want you to think that Rambus (at 800 MHz) is four times as fast as PC200 DDR. So, we went with a number that reflects the MODULE bandwidth. Since DDR DIMMs are eight bytes wide, the designation becomes PC200*8 = PC1600 and PC266*8 = PC2100. So, to repeat, PC1600 uses both edges of a 100MHz clock, and PC2100 uses both edges of a 133MHz clock. And now, you're only a little less confused than me!

Not sure if this really answers your question....I really don't remember it off the top of my head, but I can do a little research tomorrow and let you know if you're interested in more info.

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