Short Version
The type of memory the card uses. Types include SDR & DDR SDRAM, SGRAM, VRAM, and EDO. Nearly all current cards will use some form of DDR SDRAM.
Long Version
All video card memory is RAM, or 'random access memory'. Over the years there have been a bunch of different variations of ram used to try to get higher performance and lower costs. Early on (the mid 90s) there were several different types of ram used including VRAM and EDO. By the end of the 90s everyone had pretty much standardized on SDRAM.
Since then, DDR memory has been the big deal. DDR stands for 'double data rate' and means that the memory can be accessed twice during every clock cycle. This translates to double the effective memory speed. Since DDR was first used on the GeForce 1, it has been an evolutionary process. DDR2 was introduced with the GeForce FX 5800 Ultra, and later GDDR3 was introduced on the GeForce 6800 Ultra (in each case ATi followed suit very shortly after nVidia).
DDR, DDR2, and GDDR3 are all just double data rate SDRAM. But the specifications for voltages and timings were adjusted with each specification to allow for higher clock speeds (and thus higher performance). In general, the only real difference between DDR, DDR2 and GDDR3 is the speed at which they run.
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For example, I have seen a GeForce 6800 GS with 128MB DDR @ 720MHz gets 3800 points in 3DMark05; in comparison, my 6800 GS with 256MB DDR3 @ 1000MHz manages 5000 points in the same benchmark. A lot of that is due to the extra RAM on my card, but the extra 280MHz memory speed gives my card a lot more bandwidth and thus, better performance.
In general, if given a choice between a card with DDR/DDR2 memory and DDR3, *always* go for the DDR3 model. It may cost a bit more, but that card will run faster and will have much more capacity for overclocking.
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