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SDR SDRAM Typical clock frequencies are 100 and 133 MHz.  Originally simply known as SDRAM, single data rate SDRAM can accept one command and transfer one word of data per clock cycle. Chips are made with a variety of data bus sizes (most commonly 4, 8 or 16 bits),   but chips are generally assembled into 168-pin DIMMs that read or write 64 (non-ECC) or 72 (ECC) bits at a time.

Double Data Rate SDRAM Typical DDR SDRAM clock rates are 133, 166 and 200 MHz (7.5, 6, and 5 ns/cycle). While the access latency of DRAM is fundamentally limited by the DRAM array,  has very high potential bandwidth. the supply voltage was reduced from 3.3 to 2.5 V. DDR SDRAM is not backwards compatible with SDR SDRAM.

DDR2 SDRAM DDR2 SDRAM is very similar to DDR SDRAM but doubles the minimum read or write unit again, to 4 consecutive words. The bus protocol was also simplified to allow higher performance operation.  Typical DDR2 SDRAM clock rates are 200, 266, 333 or 400 MHz (periods of 5, 3.75, 3 and 2.5 ns)

DDR3 SDRAM DDR3 continues the trend, doubling the minimum read or write unit to 8 consecutive words. This allows another doubling of bandwidth and external bus rate without having to change the clock rate of internal operations, just the width

Mobile SDRAM Mobile DDR (also known as mDDR, Low Power DDR, LPDDR, or LP-DDR) It is a type of double data rate synchronous DRAM for mobile computers. non-abbreviated: Low Power Double Data Rate Synchronous Dynamic Random Access Memory.