DDR SDRAM (double data rate synchronous dynamic random access memory) is a class of memory integrated circuits used in computers. It achieves nearly twice the bandwidth of the preceding [single data rate] SDRAM by double pumping (transferring data on the rising and falling edges of the clock signal) without increasing the clock frequency
With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.
DR SDRAM for desktop computers DIMMs have 184 pins (as opposed to 168 pins on SDRAM, or 240 pins on DDR2 SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two).
DDR for notebook computers SO-DIMMs have 200 pins which is the same number of pins as DDR2 SO-DIMMs. These two specifications are notched very similarly and care must be taken during insertion when you are unsure of a correct match.
DDR SDRAM operates at a voltage of 2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce power consumption. Chips and modules with DDR-400/PC-3200 standard have a nominal voltage of 2.6 Volt.
Like the ordinary SDRAM, DDR can be also classified into two types: 200MHz (100MHz DDR) PC1600 SDRAM and 266MHz (133MHz DDR) PC2100 SDRAM.
The memory modules exterior was also a bit modified. Although the DDR modules size remained the same, they have more pins - 184, while the regular PC100/PC133 SDRAM DIMM modules have only 168 pins.Besides a larger number of pins, new DDR SDRAM DIMM modules feature lower voltage, 2.5V, which helps to reduce EMI (compare with 3.3V by the regular SDRAM). As for the latency, it is almost the same as by PC100/PC133 SDRAM modules and much lower than by RDRAM.
In addition to speed, DDR2 and DDR3 were developed to run at lower voltages than regular DDR RAM, resulting in less power needed to run memory. DDR used 2.5v. DDR2 runs off 1.8v and DDR3 1.5v.