Registered memory has extra circuits to buffer the current memory page on chip for more stable operation (or something like that). Its primarily for server and workstation usage where stability and data integrity are more important (the important part of the difference).
Agree with Fish: Think business/professional systems with registered/ECC memory. If you don't know what it is, then you more than likely don't need it. That being said - what are you looking at building/upgrading? Budget?
Recommend you read the FAQ in the memory forum for more details on RAM - it's a very good/informative read.