Myths and Legends are stories that are often very ancient, and with both of them, you can't know whether they are true. Myths are stories that are very much up to your interpretation, and they often deal with more complicated things than Legends do. Legends often have to do with people or animals, such as the legend about the Loch-ness monster. Also, Legends often seem more believable than Myth. Though there is a lot of overlap, I think that these two terms are very different.

Fairy Tales are often about magical or mystical things--things that don't seem possible. Fairy tales are meant to come alive in your imagination. Fables are often more mature, and have a lesson to learn from the story, but they are also stories meant to be enjoyed. I think that Fairy Tales and Fables are often the same thing.