What's the difference between DDR and DDR2 RAM?

Answer 1

Like DDR before it, DDR2 cells transfer data both on the rising and falling edge of the clock (a technique called double pumping). The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the speed of the memory cells, allowing transfers from two different cells to occur in the same memory cell cycle. Thus, without speeding up the memory cells themselves, DDR2 can effectively operate at twice the bus speed of DDR.

However, the price is affected and as nice as it may sound technically, the different is not that great, I would even say unnoticable if you just surf the web with your computer.

Answer 2

Design, speed, voltage and temperature.