What is the difference between SDRAM and DDR?

Single Data Rate (SDR) SDRAM has been superseded by faster Double Data Rate (DDR RAM)
DDR RAM with a Double Data Rate is achieved this by “double pumping” (transferring data on both the rising and falling edges of the clock signal), hence double the data rate. The other differences include : number of pins, voltage, speed and latency.

SDRAM will not fit or work in a DDR system and vice-versa.