A myth is an occurrence that takes place in an ante-historical past, in a time that is not our own and has little relation to the natural laws we understand. A myth attempts to explain why the world we know has come to be, whether on the scale of the Earth's creation, or of why a certain mountain range exists, or why people are a certain way.
A legend is an occurrence in the historical past (regardless of its truth or not), and attempts to explain the occurrence itself, the persons involved, and why it may still hold relevance to us. (Legend says this house is haunted because...)
A folk tale (tall tall) is an historical story. Unlike myth and legend, a folk tale is always in narrative form. A folk tale is always regarded as fiction, and as its purpose is to impart common sense, morality, or social values, the veracity of the story is unimportant.
At least this is what I have come to understand.