[Joe]

Here is Crucials Def of it:

"DDR and SDRAM can be unbuffered or registered. EDO and FPM can be buffered or unbuffered. Buffered modules contain a buffer to help the chipset cope with the large electrical load required when the system has a lot of memory. Registered modules do not have a buffer but do contain a register that delays all information transferred to the module by one clock cycle. Buffered and registered modules are typically used only in servers and other mission-critical systems where it is extremely important that the data is properly handled."

Registered will NOT work in Mobo's that dont accept it, and UnBuffered will NOT work in a registered board.