2019 Hugo Award Winners Include a Fan Fiction Site and 'Spider-Man: Into the Spider-Verse'

https://entertainment.slashdot.org/story/19/08/24/0348225/2019-hugo-award-winners-include-a-fan-fiction-site-and-spider-man-into-the-spider-verse
So what exactly is the difference between a Novella and a Novelette?