Thanks for sharing that video @Missa! That’s just what we needed! 
My takeaways:
Selenite can be any crystalized form of gypsum. Satin Spar is specifically the fibrous type of gypsum. “Selenite”, however, is commonly used for the transparent variety of gypsum.
Satin Spar itself is a “description”. There are other types such as Satin Spar Calcite or Satin Spar Orgonite, as mentioned in the video.
Another interesting thing is that if you get it wet, Selenite is not going to dissolve. It can sit in water and it’s not going to be ruined (according to the video).