DDR SDRAM, or simply DDR, is the acronym for Double Data-Rate Synchronous DRAM (SDRAM).

DDR is a memory technology that is considered 'evolutionary' as it is based upon the stable and mature PC133 or PC100 SDRAM technology.

A 100MHz SDRAM chip handles a single memory operation per clock cycle; its data rate is effectively 100MHz x 1 or 100MHz. Similarly, a PC133 SDRAM chip has a data rate of 133MHz. PC100 and PC133 are in effect 'Single Data-Rate' SDRAM.

DDR memory chips can do 2 operations during a single clock cycle. A 100MHz DDR memory chip's data rate is thus 100MHz x 2 or 200 MHz. A 133MHz DDR memory chip has a data rate of 133MHz x 2 or 266MHz.

Technically, the DDR memory bus runs at memory-bus clock rate of 100 MHz for PC1600, 133 MHz for PC2100, 166 MHz for PC2700 and 200MHz for PC3200. However, each DDR memory module and memory chip run at an effective (data) rate of 200 MHz, 266 MHz, 333 MHz and 400MHz respectively. The computer industry has adopted a practical convention of just referring to the data rate as the DDR DIMM speed. So, PC1600 DIMMs are said to run at 200 MHz, PC2100 DIMMs at 266 MHz, PC2700 DIMMs at 333MHz and PC3200 DIMMs at 400MHz.

JEDEC has defined the specification for PC1600, PC2100, and PC2700. Other specifications, such as PC3200, were originally defined by DRAM manufacturers, chipset, and motherboard manufacturers, and then were to be finalized by JEDEC.

Because of demand from computer enthusiasts, gamers, and selected system builders, Kingston has launched faster memory modules under the HyperX brand. The specifications for these modules were defined by major chipset and motherboard companies.