CD/DVD Duplication in it's purest definition means to copy discs. Actually, there are two different ways to make copies of a disc, Replication and Duplication.
Replication - the process of "stamping" your data into an injection molded disc. These discs are like the kind you get at the music or video store and have a silver look to them. The silver look is because of the aluminum that is used to reflect the laser that is used when you play your disc.
Duplication - using a disc recorder(s) to "burn" your data into a pre-manufactured write-once discs. These discs, called Recordables (CDR or DVDR), are blank discuss and cost between $.20 and $.75 depending on quantities and quality.
Quality - there are some people that will tell you not to duplicate audio or video but only replicate. Others say that there's no difference at all. They both can't be right.... can they? Well, there are differences and sometimes they do have an impact. The reality has more to do with the play back equipment than the manufacturing process. Older "set top" DVD players (hooked to your TV) occasionally have problems playing duplicated discs but rarely have problems with replicated discs (rarely doesn't mean never). Other people believe that a replicated audio disc is better than a duplicated disc. This time I don't think there's much of a difference but they may look a little different. Remember, this isn't an analog tape, digital music is digital music no mater how its manufactured.
Summary - If you need less than 5,000 discs and you need them in a hurry, duplicate! Larger numbers might better handled with replication if the normally longer turn times will allow it. It might be best to contact a company that can handle either option for you. That will help insure that you get unbiased assistance with your project.