The only thing I'd add to Doug's very nice description is that most people over 40 (a lot of people with large record collections these days) or so cannot hear the very high frequency on records anymore (above 12-14 khz or so I believe) in that we all lose that ability to some extent as our ears age. So the ear increasingly becomes a less reliable tool for determining the absolute quality of a record. if you do not believe me, play a test tone record or CD or do some other equivalent test of ability to hear different frequencies and see (hear) for yourself.
The good news is if you can't hear it, it may not matter if a record is in 100% perfect condition anymore. It will still likely sound very good!
The good news is if you can't hear it, it may not matter if a record is in 100% perfect condition anymore. It will still likely sound very good!