Memory comes in two main categories: SDR (single data rate) and DDR (double data rate), which transfers twice the data per clock cycle. Single data rate memory has long been obsolete as far as graphics cards go. Because DDR memory does twice the work of SDR memory, it is important to remember that all DDR memory usually is clock speed-advertised at double its physical clock speed. For example, DDR memory is considered to be "1000 MHz DDR" memory (a.k.a. "1000 MHz effective") when its actual clock speed is 500 MHz.
For this reason, many people are surprised when their graphics card's 1200 MHz DDR memory is reported to be running at 600 MHz. But this should not be a cause for alarm, as it's simply how DDR memory clock speed is reported. DDR2 and GDDR3 memory works on the same principle, at double the clock speed. The difference between DDR, DDR2 and GDDR3 memory is only manufacturing technology. DDR2 can generally run at higher speeds than DDR; and DDR3 can generally run at higher clock speeds than DDR2.
Memory Clock Speed
Like the graphics processor, the graphics card memory works at a clock speed that is measured in megahertz. Similarly, increasing the clock speed will have a direct impact on memory performance. As such, the memory clock speed is one of the numbers used to compare memory performance. For example, assuming all other factors (like the memory bus width) are equal, when comparing a graphics card with 500 MHz memory and a graphics card with 700 MHz memory, it is reasonable to assume that the card with 700 MHz memory will be generally faster at memory operations.
Once again, clock speed isn't everything. A 700 MHz memory component with a 64-bit bus will perform slower than 400 MHz memory with a 128-bit bus. A 400 MHz clock speed on a 128-bit bus is roughly equivalent to 800 MHz on the 64-bit bus. It should be noted that the graphics processor speed and memory speed are completely separate, and are usually run at different settings.