SDRAM as it's properly called is in it's current form of DDR3 the standard for desktop memory. It is the fastest type of system Ram that you can currently buy until DDR4 comes out.
The previous type of Ram was DDR2 and naturally in it's time the fastest available until DDR3 was released.
There are of course some differences between the two types both physically and performance wise. As we have already mentioned the speed difference (performance) is what most people are interested in since they want their computer to perform better. With Processors , video cards and hard drives getting faster and upping the computers performance the need for the memory to keep pace was apparent.
Seeing the difference would be the first step and the best way besides the package label is to look at the bottom of the Ram stick for the position of the "notch"
This photo shows a DDR and DDR2 Ram stick and the notch appears to be in the same place so the motherboard could use the same slot style and just make changes in the bios as to whether or not it will accept DDR or DDR2.
In this photo you can see that with DDR3 the notch has been moved to slightly off center thus making it impossible for it to fit into a DDR2 Ram slot.
Now that you can see the physical difference because of the "notch" you can determine by looking at your ram sticks and the ram slots on the motherboard what type of ram you have. You can also see if you were lucky enough to have purchased your motherboard close enough to the DDR3 release date to have gotten one of the boards that would accept both DDR2 and DDR3 Ram.
These two photos show a motherboard that can accept both DDR2 and DDR3 Ram, one board has 4 Ram slots and the other has 8 Ram slots making it easy to choose which Ram type you want to go with depending on budget.
Final difference between the two is that the DDR2 is limited to Double Data rate (DDR) and the speeds that were able to be set in the memory chips.
DDR3 however progressed to where it not only had DDR but it now it has Dual Channel, Triple Channel and Quad Channel. All of these refinements and advances improved the performance end of the DDR3 and made it the standard of todays consumer computer. It also made it necessary to browse through your motherboards specifications to verify what Ram is good for your motherboard.