I am not quite sure what you are asking in reference to a literary term but the way that I perceive the following is as follows.
A folktale is a "tradition." Folktales are stories that are passed down from one generation to the next. They might be humorous or serious. The characters are often familiar and well developed and in cases of stories told about animals, the animals are usually personified.
A legend is an "epic" about something or someone who is seen as larger than life. His/her accomplishments or actions are usually differed from the other ordinary members of a population.
A myth is a "story" that defies natural explanations.
A fable is a "tale" that has a moral hidden within the story.
I like the answer above, but would add a few elements. Myth, legend, fable and folktale can usually all be considered didactic (they provide a lesson or 'moral').
Additional facets seperate these terms. Myth can be traced back to a former belief or outdated religious practice. When I teach these in class there's always an awkward moment when someone asks about personal/current religious beliefs. I always do my best to reassure them that unless they're roasting offering to Zeus in their free time - no worries.
Often legend and fable walk a fine line when it comes to tracing their lineage, but a convenient distinction lies in what they strive to impart. Prolonged epic tales filled with heroic feats helping to overcome seemingly insurmountable obstacles define 'legend'.
Fables tend to take place in unidentified periods of time and locations. The presence of animals with human traits and tales geared towards a younger audience embody the classic fable format.