As their name states, SD ram is single data rate, whereas DDR is double data rate. So each clock cycle, DDR transmits twice the data. Usually that comes at the expense of latency (i.e. SDR typically would have lower latencies).
Sorry I should amend that. SDR stands for synchronous dynamic random access memory. But the rest of the info is correct.
SDRAM stands for Synchronous Dynamic Random Access Memory. DDR is short for "DDR SDRAM" and stands for Double Data Rate. Nowadays to avoid misunderstandings SDRAM is often specified as SDR SDRAM and SDR stands for Single Data Rate by analogy with DDR.
SDR SDRAM stands for single data rate synchronous Dynamics Random Access Memory and DDR SDRAM stands for double data rate synchronous Dynamic Random Access Memory.The basic difference between SDR and DDR memory is speed.Data transfer rate of the DDR is twice of the SDR.The pins on the SDR are 168 and on DDR are 184. SO DDR provides better performance compare to SDR.
I find that DDR2 really isnt that much more expensive than DDR. Basically DDR stands for Double Data Rate SDRAM. The name says it all. It was built to run with double the data rate of traditional SD RAM.