Original: The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the rate of the memory cells, so four bits of data can be transferred per memory cell cycle.
Simplified: Comparisons The main difference between DDR and DDR2 modules is that the bus on which the DDR2 memory modules are working is clocked at twice the speed of the memory cells. In practical terms we can say that four words of data can be transferred during one memory cell cycle.