DDR2 RAM or DDR2 SDRAM is a type of computer memory. It stands for: Double Data Rate 2Synchronous Dynamic Random Access Memory. So what does that mean?
Dynamic Random Access Memory stands for DRAM or Dynamic RAM which is the more common memory found in a computer. The reason its synchronous dynamic RAM is because the RAM is synchronized with the processor. The Double Data Rate 2 is self-explanatory it just means that the RAM transfers twice as much data as the SDR (single data rate) SDRAM does. DDR2 is more improved then DDR or DDR1 because it operates the external bus twice as fast as normal DDR RAM.
How can you tell the difference between DDR2 SDRAM and the other types of RAM such as SDR SDRAM and DDR SDRAM? Well DDR2 SDRAM has only one notch (slightly more to one side) and it differs from DDR RAM because it is a thinner notch and they are not in the same spot. What this means is that the memory slots in the motherboard must have one notch so that the DDR2 SDRAM can fit into it. The picture below demonstrates that.
DDR2 RAM (Double Data Rate) which means it can transfer double the data without increasing the clock speed. Typical clock speeds were 100Mhz, 133Mhz, 166Mhz, 200Mhz and 266MHz.
Megabyte (MB) and Gigabytes (GB) measure the size of DDR SD-RAM. They vary in size from 256mb-2Gb. The common sizes are 512mb, 1Gb and 2Gb. DDR SDRAM and DDR2 SDRAM have both increased the speed and size even further and they are similar prices so if possible go for DDR2 or DDR3.