DDR RAM or DDR SDRAM is a type of computer memory. It stands for: Double Data Rate Synchronous 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 is self-explanatory it just means that the RAM transfers twice as much data as the SDR (single data rate) SDRAM does.
How can you tell the difference between DDR SDRAM and the other types of RAM such as SDR SDRAM and DDR2 SDRAM? Well DDR SDRAM has only one notch (slightly more to one side) and it differs from DDR2 RAM because it is a wider 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 DDR RAM can fit into it. The picture below demonstrates that.
DDR RAM (Double Data Rate) which means it can transfer double the data without increasing the clock speed. Typical clock speeds were 100Mhz, 133Mhz, 166Mhz and 200MHz.
Megabyte (MB) and Gigabytes (GB) measure the size of DDR SD-RAM. They vary in size from 128mb-1Gb. The common sizes are 256mb, 512mb and 1Gb. DDR2 RAM and DDR3 RAM have both increased the speed and size even further and they are similar prices so if possible go for DDR2 or DDR3.