Hi there, Thank you is standard, could be said to anyone, young old, familiar, unfamiliar. It also expresses greater gratitude than a simple thanks. Thanks is more informal, too. So if someone offers you a homemade cookie, you can say simply thanks, especially if you know this person. Or it`s a stranger but the same age, let`s say. But if someone donates their kidney to you (just making a point) you can`t just say Thanks.

Thanks is quick and informal. It's what you'd use with friends to show gratitude for something unimportant. Thank you is the normal way of showing gratitude and can be used for generally anything.