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.
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.