What is DDR and DDR2 Ram?

DDR and DDR2 are incompatible specifications for RAM (DDR= double/dual data rate). DDR2 is faster (more bits per second), lower voltage. Initially Intel came out with DDR2 hardware closely followed by AMD. Already DDR3 is out but uptake has been limited due to high cost.

All DDRx RAM architectures are forms of SDRAM (synchronous dynamic RAM), i.e. DRAM with a synchronous interface. The "dynamic" aspect of this kind of RAM is due to data being stored on capacitors which eventually lose charge, so this memory is constantly being reused. It's a computer's short-term memory.

DRAM 101

Everyone knows intuitively that RAM is computer "memory" that allows "random access" (access any location in memory in any order in constant time). RAM normally refers to volatile memory where information is lost when power is turned off.

But what is the difference between dynamic and static RAM? In static RAM, an electronic component known as a flip-flop (aka bistable (Two State) multivibrator) stores 1 bit of memory, in DRAM this bit is stored instead as charge on a capacitor or transistor gate.

The physical implementation of flip-flops is covered here.