It's not a formal word; you can tell this based on its origin. Nowadays, "folks" is used in a homely, colloquial sense to mean "people in general". Sometimes it is used to mean "parents/relatives", but we also have many other slang words: the fam, the olds, the rellies...
If you use "folks" to address a group of people, you'll sound like a boy from the countryside! Some speakers manage to use this in a friendly and charming way, but as with most colloquial phrases, it just doesn't work automatically.
This may be worth a read: