Originally mastering referred to the process of transferring music from analog tape to a metal disc format used for stamping out vinyl copies. It involved the sequencing of songs, the setting of volume levels, touches of equalization and some limiting/compression. With the introduction of CDs the term grew to include the transfer from analog to digital formats. Today mastering has come to mean virtually anything having to do with the finalization of a record in any format.
There's only two good reasons to remaster an existing recording. Either you're changing the final consumer format or you think you can make it sound better than previous editions of the record. If possible the remastering engineer will go back to the original master tapes (not the original multitrack recordings, but the final two track mix) from which they'll make a working copy. Sometimes the originals are not available and a second generation or later tape must be used. There are even cases where no tape masters can be found and the engineers have been forced to use off the shelf vinyl discs as their source material. Obviously, the farther away you get from the original master tape the worst the sound quality. A remastered version can sound better than an earlier version if they are able to access earlier master tapes, use superior mastering equipment and/or have greater skill at mastering than the previous engineer. The remaster can also sound worst if these three conditions are not met.
