Physical Differences Between Different Types of Memory
You should think of the different types of RAM - DDR, DDR2, DDR3 - as different "generations" of memory module. For instance, DDR2 SDRAM is twice as fast (with regard to clock rates and bandwidth) than the earlier DDR SDRAM
When selecting RAM modules for your motherboard, you should be aware that different memory types cannot be used on all motherboards.
Naturally you're not going to confuse a laptop memory module with a full-sized desktop computer RAM component, but there are other differences. When inserting a new memory module, it is imperative that you get the component the right way around - this is because there is a notch around two-thirds of the way across the length of the RAM.
As you might have worked out, this forces you to insert the module in the right way, but not all RAM modules have the notch in the same place. You can see in the accompanying illustration that DDR, DDR2 and DDR3 modules all have the notch in different positions, and this is done to prevent the wrong modules being installed.
Sadly, it isn't unheard of for inexperienced users to try to force in a memory module that looks as though it might fit, with the resulting damage to the RAM and the motherboard rendering each unusable.