What are “unbuffered” and “registered” (reg) memory modules ?

UNBUFFERED – No buffer the memory is connected directly to the chipset controller. Memory modules that are used in desktops or notebooks are mostly unbuffered.

BUFFERED – A buffer is used to help the system control large amount of memory.

REGISTERED – Registered modules do not have a buffer but do contain a register that delays all information transferred to the module by one clock cycle, this increases reliability and the main application is for servers.