DDR-:Double Data Rate,
SDR-:single Data rate,DDR transfers data twice the speed of
SDR.

DDR is double data rate...
SDRAM is synchronized dynamic Ram....

the difference between both is how the data is stored and 
transferred by them.

SDRAM stands for Synchronous Dynamic Random Access Memory
DDR-SDRAM stands for Double Data Rate Synchronous Dynamic
Random Access Memory

The difference between SDRAM and DDR is that SDRAM transfers
data on the rising edge of the clock signal. However, DDR
transfers data on both the edges of the clock signal ie., on
the rising and the falling edges of the clock signal. As a
result, the data transfer rate of DDR is faster than SDRAM.
It is almost twice the speed of the SDRAM.