terminology question

Hello,

I'm just trying to understand the terms 
  "representation" and "variant".
  
After reading the spec for a while I suspect that
every possible entity from a given resource is a
"variant" and if content negotiation comes into play
it is termed "representation" as well. 

If I have a document that changes only over time (eg CGI output), then
are the different entities termed as different "variants"
of the same resource as well? Or is it only sensible at a given
instant? Is there any practical explanation available?

Any help is appreciated very much.

Thanks a lot in advance,

Jacob