On these forums I repeatedly see references to "stems". Please forgive me, but what exactly is a stem? I assume it is a track of audio. If this so, then why is it called a stem, and not a track? Is this something basic I am missing? I truly don't know.

Your assumption is right, stems are the original unprocessed audio track.
 
I'm not sure why they're called stems as opposed to tracks but it's either because tracks might not necessarily be "unprocessed" whereas stems are,  or it's just that tech people are like lawyers and like to have a magic language to baffle us lesser beings
 
 
Steve

A stem is basically a sub mix of similar instruments.  What you might find in busses.
 
Stems are mostly used when remixing, rather than having to recreate track FX from scratch. So you may have a guitars stem, a Vox stem, drums stem, etc and then a few individual tracks like lead vox, lead guitars, etc.
 

ok,  this is interesting.  What I posted is the exact opposite to what Steve posted...

I was just about to say that karyn
 
You may actually be right but I thought they would be the, 'unprocessed' audio.
 
I took the stems from an engineer who mixed my bands single last year and as far as I know there were no effects on them.
 
 
I think you're explanation is closer
 
Google Stems
 
 
Steve

Have to agree with Karyn.
http://en.m.wikipedia.org...m_mixing_and_mastering

This from Wiki explains the way I see stems perfectly.
 

Wiki
In music mixing for recordings and for live sound, stems are subgroups of similar sound sources. When a large project uses more than one person mixing, stems can facilitate the job of the final mix engineer. Such stems may consist of all of the string instruments, a full orchestra, just background vocals, only the percussion instruments, a single drum set, or any other grouping that may ease the task of the final mix. Stems prepared in this fashion may be blended together later in time, as for a recording project or for consumer listening, or they may be mixed simultaneously, as in a live sound performance with multiple elements.

 
Had I written the Wiki entry, that's exactly what I would have put.  or tried to put...

Karyn is right. It comes from the movie industry not audio. The idea of stems is that they will have dialog, sound FX and background music. Stems are not the raw audio tracks but sub mixes of various similar mixed content that has a natural relationship. A dialog stem for example may include all the miced characters plus any voice overs and the like.
 
There maybe theme music plus dramatic music to add emphasis to a scene.  All of this will be presented to a film editor for mixing with the movie. They wont want to use all the original tracks but rather sub mixes of like content.
 
 
Its used here as simply a sub mix created by using multiple buses. Each sub buss is a stem. 
 

Oh thank you all. I suppose that makes sense. Makes me feel a little smarter now.

Stems are common in mastering if the mastering engineer isn't happy with the mix, but doesn't want to remix it. For example I might ask for stems of the drums, vocals, bass, and mix of everything else with those elements dropped out. A lot of times I'll ask for a track without a lead vocal and separate vocal tracks, both with and without processing.