Every computer needs RAM to function. But did you know that pretty much all the RAM we use these days uses some clever tricks to run much faster than otherwise would.
Even older RAM like DDR3, uses these techniques. In fact, it's right in the name, DDR stands for double data rate, which means just what it sounds like it means. It can move data twice as quickly as single data rate memory, which hasn't been in our computers for very long.
The original DDR actually came out way back in 2000, not to be confused with the other DDR which came out in 1998. Unfortunately, I don't have a stick of the original DDR to show you, but I can show you a bit about how it works.
So this is an example of a simple clock signal that your computer uses to keep components inside synced up. 
Conventionally, one piece of data is sent per cycle or for each one of these humps, but you see how each cycle has a rising and falling edge? Well, DDR sends data on both the rising and the falling edges, which works because your memory modules can detect at what point the internal clock is in its cycle.
But hold on a second, this would only be double the data rate over a ancient school single data rate RAM. So how are we getting generational improvements like DDR2, 3, and 4, which continue to increase the data rate by a factor of two each time?
It turns out that there are actually two clocks involved in what makes DDR RAM work. The external bus that connects your RAM to the rest of the system and an internal memory clock. For DDR2, the internal clock was kept the same, but the external bus clock was doubled. This means that the external bus can carry twice as much data per unit of time.
To take advantage of this, the internal memory array, where the data is actually stored, doubles the number of bits sent at one time to the external memory bus. 
This process is also called prefetching and it's responsible for why DDR2 is faster than DDR and why DDR3 is faster than DDR2. The external bus's faster clock can then carry the extra data and boom! You've got faster memory.
Advances in manufacturing that keep signal integrity high have allowed newer versions of DDR to operate at these higher speeds without a hitch.
It should be noted, however, that DDR4 works slightly differently in that memory arrays are divided into bank groups, which means that prefetches can be pulled from different parts of the RAM at the same time leading to higher speeds than DDR3.
Boosting the external clock is also part of how newer versions of DDR can run at faster clock speeds while drawing less power because the internal memory array runs at speeds slower than the external bus and sends more prefetch data per cycle. It keeps power requirements from getting too high, which when combined with smaller die sizes means new DDR generations don't need as much voltage.
Of course, unlike many other generational improvements, one thing that newer versions of DDR do not feature is backwards compatibility.
This is a stick of current gen DDR4
and you'll notice that stick of DDR3, that the notches are in different places. That means that these two sticks cannot be used in the same motherboard.
It's made this way on purpose since the different generations of DDR have different electrical and signaling requirements, that or it's just a plot to make all of us buy new memory every couple of years. Either way, I'll cough off whatever they want as long as it has RGB.
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