What does "registered" and "unregistered" mean with regards to memory?

[Seeko]

And which one is better and why?

 

[John]

From Crucial.com:

The term "registered" refers to how the memory module processes signals. Registered modules contain a register that delays all information transferred to the module by one clock cycle. This type of memory is primarily used in servers and was designed for modules with 32 or more chips on them to help ensure that data is properly handled.

While most PCs will only accept unbuffered SDRAM, there are some that accept registered SDRAM. Keep in mind that when you install registered SDRAM, all of the modules installed in your PC must be registered because unbuffered and registered modules are not interchangeable.

For home users and "most" people, non ECC and unbuffered is the way to go.