[System Memory] What is the difference between DDR2 memory and DDR3 memory?

The current industry standard is DDR2 memory running at from 667MHz to 800MHz. But there are upper class laptops such as the Apple's Macbook Pro series that utilizes a DDR3 class memory which currently clocks/runs at 1066MHz. A DDR3 memory generally runs faster than DDR2 memory and can achieve a wider bandwidth (Think of a wider freeway with a higher speed limit).

Expanding on this topic: Is DDR3 class memory really better than DDR2 class memory?
Comparing a 800MHz with the 1066MHz memory, as an End-user you might not notice the 266MHz increase in computing speed. In fact there is not much gain in performance (up to 3% gain) comparing the 800MHz memory with 1066MHz memory of the same size.

The 3% performance increase definitely varies by usage, most day-to-day stuff that you might do (such as Hulu or browsing your photo libary) will only takes advantage of the memory size other than the memory speed. And I can't think of one that only takes advantage of the memory speed.

Applications that are intensive on size: Streaming videos online or burning a DVD
Applications that are intensive on speed & size: High-End gaming that requires fast graphic rendering, research and 3D-Modeling.

Conclusion:
If you like gaming, it is obviously that a faster clocked and a larger memory can improve your computing performance. But comparing DDR2 class memory with DDR3 class memory of the same size there is a very little advantage and is probably not worth the marginal cost (If the sales told you "DDR3 class memory has a big advantage over DDR2 class memory") I recommend to look for a computer with a larger sized memory other than a faster clocked memory because in general DDR3 memory cost more than DDR2 memory.