No difference! “Autumn” is generally used in the UK, while “Fall” is used in USA. They both mean the same season. I’d say because of TV and Film consumption, people are starting to use either, more so people in the UK using the word “Fall”.
You could use the word “autumnal” to describe weather as well which just denotes Autumn/Fall weather. Furthermore, “autumnal” could be used to describe colour palettes i.e browns, oranges, reds. Hope this helps!