Rebuilt, remanufactured, and reman parts all mean basically the same thing. Used OEM parts are taken apart, cleaned, worn out pieces are replaced, and then the part is put back together so it meets or exceeds the original OEM specs. As this process reuses the main components of the part, for example, the engine block, or the shell of a caliper, it is often cheaper to do this process for resale than it is to manufacture an entirely new part. This is why rebuilt auto parts are common because they are OEM parts that are like new for a lot less than the new OEM part would be from the dealer.