Use "folk" when you are referring to a group of people sharing a common identity, even if there are a plural number of people in that group,
E.g., "We ladies should just let the men folk worry about the election." (Obviously there is more than one man, but we are referring to men as a collective group here).
Use "folks" (in the plural) when addressing a group of people who may not necessarily share a common identify: Basically, use "folks" when you can replace the word with "people."
E.g., "I chatted with the folks at the farmer's market." (You can say: "I chatted with the people at the farmer's market.")