Double Data Rate Random Access Memory, or DDR RAM is what all computers use. Graphics Double Data Rate or GDDR is what graphics cards use. Each version of DDR gets faster and faster. Current Graphics cards are using GDDR 5, 5X, 6, or 6X. Figuring out which cards have which memory installed is not actually a big deal though. It is the Graphics Benchmark of the chip which we can judge cards by. If two cards have the same chip, then the one with the larger RAM is better, but it’s hard to tell exactly by how much.
If two video cards have different GDDR versions, the one with the higher version number is definitely better RAM. Video cards are, again, to be judged on the chip first, because you could easily have a video card with GDDR5 be faster than a different card with GDDR6 but a much less powerful Chip.