You are asking about an entire universe of scholarship and centuries of study, so answering your question with reddit-precision is difficult. I recommend James M. McGlathery, ed., The Brothers Grimm and Folktale (1988) and Jack Zipes, The Brothers Grimm: From Enchanted Forests to the Modern World (2002) (really, anything by Zipes is worth your time).

Please keep in mind that the Brothers Grimm offered distillations of folktales that they recorded, publishing them in abridged and often sanitized form for children. The original folktales were for adults - both in length and complexity and in violence and sexuality. The Finn Antti Aarne and the American Stith Thompson pioneered a classification system of folktales, which facilitates research. Archives organize their material. Researchers can easily contact folklore archives internationally to obtain all the variants, conduct comparisons, and determine the nature of variants (with a possible eye to their history). The Seven Ravens, for example, is AT Tale Type 451. And using that information, see this website for variants. - D. L. Ashliman's website is extremely useful and reliable.

Here are a few paragraphs from my Introduction to Folklore that I will e-publish in September; feel free to ask questions - I'm not sure where your enquiries will lead, but be careful about proceeding: you may become a folklorist!

European folklorists, following the lead of the folk themselves, have long recognized two forms of oral tradition, Sagen and Märchen, legends and folktales. While there are many other forms of oral tradition, legends and folktales stand in opposition to one another, yet share a great deal.

Legends – or Sagen as the profession often prefers – are short, single-episodic stories told chiefly in the daytime. More importantly, the teller intended the listener to believe the story. Legends often have horrible ending to underscore the story’s important message. A large number of them are, after all, typically meant to be instructive, to serve as warnings in some way. These types of stories are not necessarily long-lived. Their point is to reinforce and prove the legitimacy of a particular belief. Nonetheless, some legends take on a traditional character and migrate over considerable spans of time and space.

Folktales – or Märchen, again using the German, technical term – are longer stories with more than one episode. They are restricted, in theory at least, to evening presentation. A folktale is not to be believed, taking place in a fantastic setting. The European folktale also requires a happy ending, the cliché of “happily ever after.” Any given folktale can be told with considerable variation, but they are traditional in basic form, and folklorists have spent decades tracing the history and distribution of these stories.

A word here about the term “fairytale” is appropriate. At the end of the eighteenth century, various writers, most prominently the Grimm brothers, began publishing children’s stories based on folktales. These collections became extremely popular, particularly among the urban and increasingly literate emerging middle class as it found itself removed from the peasant soil that served as home to the stories. Fairytales often cause misunderstandings. In a culture that knows more about fairytales than Märchen, people assume that the folktale was intended for children. This is certainly not the case since the stories were often violent or sexual in ways thought inappropriate for children. Indeed, the telling of a folktale was usually delayed until the children had gone to bed. While fairytales provide the modern reader with the easiest access to the many stories that were once told internationally, one should always realize that they are removed from the primary inspiration. The original stories and their content provided serious entertainment for adults and they were part of an oral tradition, not something that was fossilized in writing.

The evolution of published fairytales had a profound effect on the subject of fairies, elves, trolls, and similar entities. Because fairytales became the literary domain of children, many people – including later writers – assumed the same was true of the supernatural beings. In their original context, nothing could be further from the truth. These were not cute, diminutive creatures whose sole purpose was to delight children. They were powerful, dangerous, and capable of great harm. The European peasantry feared and respected them, and their stories underscore this, conveying in uncompromising terms the code of ethics and behavior that one must employ to survive an encounter with the dangerous world of magic and power.

The definition proposed here for “fairytale” does not necessarily coincide with how people – and even some folklorists – use the term. Some scholars regard “fairytale” as appropriate for the more fantastic expressions of folktales as they were told by the folk. The reason why the term is not used in that capacity here is because the folk did not refer to these stories as fairytales and because the term implies a degree of innocence that is inappropriate; again, “fairytale” is most suitably reserved for the published children stories that gave literary expression to the adult oral fictions of the folk.