Although the technical realization may be a lot more difficult, the basic principle of DDR-SDRAM is very simple. While the new memory module is clocked at the same speed as normal SDRAM, it is able to transport double the amount of data by using the rising as well as falling edge of the clock signal for data transfers. We are all aware of this technology since AGP2x and JEDEC is already working on the DDR II spec, which will double the data transfer once more, using the quad-pumped technology known from AGP4x or the upcoming Pentium 4 bus. DDR-SDRAM has another important improvement over PC133 SDRAM. Its voltage supply is using only 2.5 V, instead of 3.3 V. This and the lower capacities inside the memory chips lead to a significantly reduced power consumption, which makes DDR-SDRAM also very attractive for notebooks.
Unfortunately DDR-SDRAM DIMMs are not compatible with the SDRAM DIMMs we are using now. The new DDR-DIMMs come with 184 instead of the 168 pins used by SDRAM-DIMMs. The module itself looks almost identical to the older SDRAM, but it has only got one notch instead of the two notches found in SDRAM-DIMMs.
The official naming is also a bit confusing. Originally the naming was PC200 for DDR-SDRAM that operates at 100 MHz memory bus and PC266 for the 133 MHz bus. After Rambus however used PC600, PC700 and PC800 for their RDRAM modules, which sounds a lot faster than PC200 or PC266 although it isn't, the memory industry came up with 'PC1600' and 'PC2100' instead. While PC200 and PC266 are only using the effective clock of the data transfer for their numbering, PC1600 and PC2100 use the actual peak data transfer rate in MB/s. Thus PC200 is the same as PC1600 (64 bit * 2 * 100 MHz = 1600 MB/s) and PC266 is equal to PC 2100 (64 bit * 2 * 133 MHz = 2133 MB/s).