Double data rate (DDR) RAM is a JEDEC-created standard in which data is transferred twice as quickly. Instead of doubling the actual clock rate,
DDR memory achieves the doubling in performance by transferring twice per transfer cycle: once at the leading (falling) edge and once at the trailing (rising) edge of the cycle. This effectively doubles the transfer rate, even though the same overall clock and timing signals are used. DDR in itself doesn't specify a type of RAM, but a technology that modern RAM takes advantage of.

SDRAM is short for synchronous DRAM, a type of DRAM that runs in synchronization with the memory bus.

So, DDR SDRAM is effectively SDRAM that has the DDR technology implemented.

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