Double data rate (DDR) SDRAM doubles the rate of speed at which standard SDRAM can process data. That means Double data rate (DDR) SDRAM gives you twice the bandwidth of standard SDRAM.
As in standard SDRAM, DDR SDRAM is tied to the front-side bus clock in your system. The memory and bus execute instructions at the same time rather than one of them having to wait for the other. As bus speeds have increased, this has improved system performance.
The big difference between DDR SDRAM and standard SDRAM is that DDR reads data on both the rising and falling edges of the clock signal. Standard SDRAM, or single data rate (SDR) SDRAM, only carries information on the rising edge of a signal. Basically this allows the DDR module to transfer data as SDR SDRAM. For example, instead of a data rate of 133MHz, DDR memory transfers data at 266MHz.