Wiktionary tells me both are underwear, calzón for women and calzoncillos for men. However, when I looked up the words on Google images (something I always do with new words to get a better idea of real-life meanings), I see a mix of underwear for both genders for either words. What gives? If it's a matter of regional preference, what are those preferences?
In Spain nobody favors the use of
calzón for feminine intimate wear. They are referred as
bragas, aways in plural even if it is just one piece of clothing. The word
braga, in singular, is a different piece of clothing: a sort of scarf.
To refer to men underwear people usually use
calzoncillos. I think that, on rare occasions, you could hear "calzones", but not
calzón to refer to men's underwear. Younger people will also favor the use of
gayumbos(for boxer type. See and example) depending on their shape to designate men's underwear, besides 'calzoncillos`.
In Chile "calzón" is for women and "calzoncillo" for men, as Wictionary says.
In recent times, it is there "calzones/calzoncillos" are used to refer to a type of outdated clothing, unlike other more modern and specific types (tanga, bikini, pantaleta, boxer, slip, zunga).
That is, if a store offer "calzones" one would expect them to be for old ladies and little girls.
However, as general concepts anyone understood that a "pantaleta" is a type of "calzón".