Differences between DDR3, FB-DIMM, and DDR4, for the relative hardware novice?

[SunPaladin]

Could somebody please offer a comparison of the differences between the desktop-based PC RAM types of DDR3, FB-DIMM, and DDR4, for the relative hardware novice?

Also: of the 3 types, which has the maximum memory capacity in terms of total RAM supported, and which is the fastest and most efficient / stable?

[geek-in-training]

DDR3 is the fastest desktop RAM currently available.
DDR4 is still in the design phase and won't be out until 2014. (It will use lower voltages to increase "speed")
FB-Dimm is "fully buffered" and is used primarily in servers (very pricey).
The capacity or total RAM that you can use is dependent on the motherboard you are using. In theory, there is no limit as to how much you could install. But there is a space restriction on a motherboard and so 4 or 6 slots are usually the max. You are now seeing 8Gb RAM for DDR3.