Stands for "Double Data Rate."
DDR is a version of SDRAM, a type of computer memory. Unlike SDR (single data rate) SDRAM, which can perform a read or write action once per clock cycle, DDR can perform two—one on the rising edge of the electrical signal and again on the falling edge. DDR also operates at a lower voltage than SDR (2.6v compared to 3.3v), making it more power-efficient.
DDR RAM is available in both DIMM modules for desktop computers, and SO-DIMM modules for laptops. DDR DIMM modules have more pins than SDR DIMM modules, as well as a unique notch position, preventing a DDR DIMM from being inserted into a SDR DIMM slot and vice versa.
DDR RAM modules are graded based on the amount of data they can transfer per second, which is based on its clock speed. For example, a PC-1600 module operates at a clock speed of 100 MHz. It transfers 64 bits, or 8 bytes, of data twice per clock cycle for a total transfer speed of 1,600 MB/s (100 MHz x 2 transfers per cycle x 8 bytes). A faster PC-3200 module instead operates at a clock speed of 200 MHz, for a total transfer speed of 3,200 MB/s (200 MHz x 2 transfers per cycle x 8 bytes).
NOTE: DDR SDRAM was introduced in 2000, and succeeded by DDR2 SDRAM in 2003. DDR is also retroactively known as DDR1 when being compared to later generations.