Please explain mono

I forgot to add, a number of "older" records, like those pre-White Album Beatles one were done in mono, the stereo versions mixed later.  Many other records from back then are in the awful "artificial stereo".  The first Buffalo Springfield album was remixed in mono by Stills & Young after their managers messed up the stereo version.  No one should ignore those records because they were in mono, unless you're just an "audio freak" caring only about sound and not music.  I don't think many of us are that.

Insofar as the mono cartridge thing goes, if you have a setup with two tonearms you are blessed.  Most aren't gonna have this, and even if you have switchable headshells most likely further setup is warranted.  Myself, I have found playing a mono record with my stereo cartridge works fine.  I guess a mono cartridge would do much to center the sound, but I've been shocked with some of my good-quality 50's mono records sounding very much like some sort of stereo!  I guess I'm not a purest, but the bottom line here is that you may not be missing much in an audiophile sense by listening to a mono record.  Certainly you may be listening to a performance that you can't hear any other way.  I just got an original copy of Schwartzkopf's 1954 LP performance of Strauss' Four Last Songs, cleaned it up, and I would never let that thing go.