SD, DDR, DDR2 and DDR3 are all different types of RAM (Random Access Memory). RAM is a kind of computer memory that temporarily stores information, improving your systems performance. Or in other words RAM simply holds data for the processor. The processor also has its own memory known as the L2 cache, the cache lies between the processor and the RAM. The processor sends and retrieves data from this cache, when this cache overflows data is sent to the RAM.
There are mainly four types of RAM; SD, DDR, DDR2 and DDR3. They differ in the way they operate and mainly in their operation speeds. All RAM's used in computers today have a synchronous interface meaning they wait for a clock signal before it can respond to clock signals.
SDRAM stands for synchronous dynamic random access memory. SDRAM is Single Data Rate meaning that SDRAM can accept one command and transfer one word of data per clock cycle. As mentioned above SDRAM's have synchronous interface therefore they depend on the computer clock signals to perform operations. Typical speeds of SDRAM are 100 and 133 MHz.
DDR SDRAM stands for double data rate synchronous dynamic random access memory. DDR RAM 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 stands for double data rate 2 synchronous dynamic random access memory.DDR2 is twice as fast as DDR which means twice as much data is carried to the module for each clock cycle. Also due to the design improvements DDR2 consumes less power as compared to the DDR memory. DDR2 speeds range between 400 MHz (DDR2-400) and 800 MHz (DDR2-800). DDR2-400 transfers 3200 MB/s. DDR2-800 transfers 6400 MB/s.
So by now after reading the above it is quite obvious what DDR3 SDRAM stands for, double data rate 3 synchronous dynamic random access memory. In theory DDR3 is supposed to act twice as fast as DDR2 memories. Thus DDR3 speeds range between 800 MHz (DDR3-800) and 1600 MHz (DDR3-1600). DDR3-800 transfers 6400 MB/s; DDR3-1600 transfers 12800 MB/s.