Difference between DDR2 and DDR3 cards

What is the difference between a DDR2 and a DDR3 memory card and can it be used interchangeably from one platform to another?

What is the difference between a DDR2 and a DDR3 memory card and can it be used interchangeably from one platform to another?
First let me tell the full form of DDR. It means, Double Data Rate which helps to double the transfer per cycle. One way to understand the differences between DDR2 & DDR3 Memory card is to define them separately, so here we go:
Can it be used interchangeably from one platform to another?
Thank you Shiv Kumar,
You really helped me to clear my doubts regarding the RAM. I am planning to buy a new pc and i am very confused about the features of DDR3 ram. This post really helped me to clear my doubts about the features of it.
Really helpful.
Thank you so much techyv.com