What does registered memory mean?

[LittleKing]

This is really a simple question, one I should know by now, but I don't. When I look at memory I know what everything else stands for but I just don't know what registered means. Can someone help me out? What am I saying, of course someone will be able to, this is sysopt.

[hover]

SDRAM memory that contains registers directly on the module. The registers re-drive the signals through the memory chips and allow the module to be built with more memory chips. Register and unbuffered memory cannot be mixed. The design of the computer memory controller dictates which type of memory the computer requires.

[LittleKing]

Would I be correct then, in saying that registered memory is better, or more efficient? If not, why? Or should I just ask what are the advantages and disadvantages of "registered memory."

LK