What is the difference between SDRAM & DDRAM?

SDR is single data rate, ddr is double data rate.

Sdr transfers the data from the memory to wherever on either the rise or the fall of a clock pulse. DDR will transfer data on both.

This doesn't meant that ddr is twice as fast, as there are overheads and other things to take into account to compare the two, but it is generally a faster scheme.

Double data rate - Wikipedia, the free encyclopedia