DDR2 was new in 2003, and started with a peak transfer rate of 3200MB/s. Over time DDR2 transfer rates became available in 4266, 5333 and even 6400. The PC2-5300 was the most commonly used once it became widely accepted. It still is used in many servers and can still be found for purchase for older machines.
DDR3 surfaced in 2007 and with it came higher speeds. There are other measurements of speed other than the peak transfer rate, such as the data rate, which is measured in MT/s as well as the I/O bus clock, but for simplicity we will stick with peak transfer rates for this article. DDR3 started out with a speed of 6400, but more commonly used would be the speeds such as 8533, 10666 and 12800. Those would be the most widely used speeds although there was also 14933 and even 17066 PC-3 or DDR3.
In 2012 JEDEC, the company that oversees technical specifications for uniformity, released the standards for DDR4. With it came new DDR4 memory with peak transfer rates including 12800, 14933, 17066 and 19200MB/s. As with the other transitions to newer memory speeds that were available in the last generation are all but unused as the higher speeds become widely accepted.
With each new generation we have differences in the keyhole, which is a slot to ensure the correct memory module is being used. Memory from the wrong generations can’t be used in machines which don’t support it, and having the keyslot ensures the wrong modules aren’t inserted.
Other variations on Server memory are unbuffered, fully buffered, or registered. Many servers can use only unbuffered, or only registered, or only fully buffered memory. The difference is how the memory reacts to the information it’s given. Unbuffered memory doesn’t generally have error checking and correcting, and is therefore not ECC. While fully buffered, and registered memory include error checking and correcting.