Used simply means you are getting it second hand, no warranties, no guaranties, if it dies 2 days after you buy it you are SOL unless its sold to you by someone who has bought a transferable service plan and it is still valid.....
A refurbished item is an item that was returned to the manufacturer for warranty repair and they replaced it to someone with a new one and repaired it to work like new. It has to meet the original working specs but the exterior can have some cosmetic issues like scratches. If you are lucky, it can even be an item that was never sold but returned to the manufacturer for a newer model. In either case, it usually has a modified or sometimes the same original warranty as when it was new.
You are probably better off with a refurbished unit then a used one, you are guaranteed some warranty service with a refurbished and nothing with used. When buying refurbished, consider the cost difference between the item and a new similar item. If you are buying something that has to be shipped, consider the shipping charges as well. You may be better off buying something new.