What is the difference between SD RAM & DDR RAM & DDR2 RAM?

SD RAM:

SDRAM is Single Data Rate meaning that SDRAM can accept one command and transfer one word of data per clock cycle. Typical speeds of SDRAM are 100 and 133 MHz.

DDR SDRAM:

DDR SDRAM transfers data twice per clock cycle hence the name double data rate. DDR clock speeds range between 200 MHz (DDR-200) and 400 MHz (DDR-400). DDR-200 transfers 1600 MB/s while DDR-400 transfers 3200 MB/s.

DDR2 SDRAM:

DDR2 SDRAM is twice as fast as DDR which means twice as much data is carried to the module for each clock cycle. Also consumes less power as compared to the DDR memory.