Is folk a noun

Yes, folk is a noun, and an adjective. As an adjective, it means "of the (common) people" - as in "Folk Music", "Folk Dancing". As a noun, it is always plural, and also means "(common) people": "Good folk live in that village". But you could also use "folks", with a plural ending, in that sentence. More often, "folks" is used when informally addressing a group of people in a friendly way - "Good evening, folks, we hope you will enjoy the show." or "OK folks, come on in and have a seat!" You cannot have one folk - it's meaning is always plural. However, there is a popular localized colloquialism: "He's (or she's) good folks" which breaks this rule, in a warmly humourous way.