DDR ram hits twice, at the top and bottom of each wave of voltage. Normal SD Ram hits only once, at the top of the voltage wave. DDR effectively doubles the speed of RAM.
DDR stands for Double Data Rate and SDRAM stands for Synchronous Dynamic Random Access Memory, so when you see DDR SDRAM advertised it means: Double Data Rate Synchronous Dynamic Random Access Memory, (now you know why they shorten it)
there was SDRAM previously, which were rated in PC66, PC100, PC133, and PC150, its 168-in RAM as DDR is 184-pin