Unbuffered memory (or unregistered memory) is RAM where there is no hardware register between the memory controller and the RAM chips. Unbuffered memory is the opposite of registered memory. Registered memory is more stable, one clock cycle slower, and more expensive than unbuffered memory. Registered memory is usually used in servers and workstations, while normal PCs usually use unbuffered memory.
Some high-end hardware, such as servers, require the use of registered memory.
In the past, registered memory was called buffered memory and unregistered memory was called unbuffered memory. Registered memory got renamed in common use, but unbuffered memory did not.
Buffered (Registered) Memory has ECC (Error Correction Code aka “error checking and correcting”) and is often but not always one clock cycle slower then unregistered memory. Unregistered memory comes in ECC and non-ECC varieties. While ECC can cause a minor reduction in performance, it can prevent costly errors when running memory intensive software. For this reason it is often used in servers and workstations where up-time and reliability outweigh the desire for a slight performance increases. On desktops, gaming, and enthusiasts computers non-ECC is often preferred because a minor speed increase is desired over a slight increase in stability.