Memory

I'm trying to understand the differences between types of memory, such as DDR, DDR2, etc. I'm looking through graphics cards on newegg, but I see cards with less memory in GDDR3, and other cards with more memory but with just DDR memory.

So could anyone tell me in laymans terms why one is preferable over the other?

GDDR is just video card memory the higher the # the better. DDR is system RAM.