DDR vs DDR2 Memory

Factual Questions
1

What is the difference between DDR and DDR2 memory? My computer uses DDR, but DDR2 is much cheaper. Is it possible to change?

2

No, they’re not interchangable. Some motherboards came with both DDR and DDR2 slots and you could use one or the other, but those were rare.

3

Thanks yoyodyne. Unfortunately, my motherboard only has slots for DDR.