Richard M. Dorson, in Folklore and Folklife begins his book by saying:
This volume is intended to answer to the questions "What is folklore?" and "What does a folklorist do?" Anyone who teaches, studies, or does research in folklore continually faces these questions, whether at cocktail parties, where he is supposed to give a sweeping answer in thirty seconds, or from friends, colleagues, parents, and new acquaintances in almost every social situation (Dorson ix).
Obviously it is difficult to give a definition of folklore. It's one of those things where the more you know about it, the harder it gets to define. I'm not going to attempt to define it here. Here are some definitions that I like, though, from: