Words like reggae, hip hop, ska etc give us an idea what we expect music to sound like. Folk,as the termused to be employed, provides no such information. The term "folk" was used to describe the way in which the music was made, and how it related to society. So it contained no prescription at all as to how the music should sound: English, Usbek and Fiji folk music naturally sounded pretty different to each other, as they were created by similar processes but in widely differing cultures. That is how I tend to use "folk" in most discussions. Obviously the word is also used as a catch-all description for "music you mighht hear at a folk festival in the UK/USA/ wherever you live), in which case it covers mostly guitar based acts and a lot of singer-song writers.
So on one occasion I might say "I am studying polyrhythms in Melanesian folk music", and on another I might say "at the moment, folk performers like Seth Lakeman seem to be getting a lot of mainstream airplay". It is perfectly usual fro the same word to mean different things in different contexts. But it can lead to strange anomalies: as in a previous post when someone put "folk" and "blues" as different categories of music.In my book, blues is a sub-category of folk. Oh dear,life is so complicated, and some people get so very angry if their favouties aren't labelled as folk by everyone.