SDRAM DDR DDR2 DDR3
Voltage 3.3 2.4 1.8 1.5
Pin Count 168 184 240 240
DDR3 is the latest standard of memory, surpassing the aging DDR2 standard.
In the past there was EDO, SD, RDRAM and DDR types of memory. Here we will focus on the most recent 4.
There are a few physical differences involved, the most important is keying.
Keying refers to the notches on the bottom of the memory module. Both the memory module and the memory slot are keyed a certain way to keep the different types of memory from being put into slots other than the intended one.
Here you can see the slot difference between DDR on the left and SD on the right:
Along with the pin count, the voltage is different. DDR2 is 1.8 volts, DDR is 2.5 volts, and the voltage for SDRAM is a whopping 3.3 volts.
Looking very closely at the memory slots th voltage is stated on each type of slot: