The Aarne-Thompson-Uther Classification of Folk Tales (ATU) is a catalog of international tale types. It is an expansion of the system first developed by Aarne (1910) for European tales and later expanded by Thompson (1928, 1961) to include more tales from Europe as well as tales from India.
The Classification of Folk Tales should not to be confused with the Stith Thompson Motif-Index of Folk Literature. The ATU catalog is a classification of the whole plot of a story into seven broad categories: Animal Tales, Tales of Magic, Religious Tales, Realistic Tales, Tales of the Stupid Ogre (Giant, Devil), Anecdotes and Jokes, and Formula Tales. The classification uses numbers 1-2000. References to the older Aarne-Thompson classification have the AT prefix. Those the Aarne-Thompson-Uther classification use the ATU prefix. The AT numbers were subsumed within the ATU classification.
The Aarne-Thompson-Uther Classification of Folk Tales is partly available online. The schema, broad categories, then subcategories, then speciifc tale is given. Click the title of the category to see specific folk tales. Click the NUMBER of the folk tale to see if a story is attached and to see variants. Many folk tales do not have the stories attached. If you are looking for a particular story, use the search box. The search box searches for stories, Puss in Boots, for example, rather than the classification scheme.
Keep in mind that the online version is incomplete. The paper edition of the Classification, available in Reference GR1 .F55 284-6 (the book cover says FF Communications edited for the Folklore Fellows) provides much more information and gives bibliographical sources.
The Stith Thompson Motif-Index of Folk-Literature, is also available in Ellis Reference GR 67 .T52 as well as online. The web page is framed. The left hand side list broad categories. Click a category and the middle pane will show the individual motifs.
An entry will look like this:
A 753. Moon as person
A753.1 Moon as wooer
A753.1.1 Moon abducts woman. Eskimo (Kodiak): Golder JAFL XVI 29.
A753.1.1 is an Eskimo folk tale and is mentioned by an author name Golder, in volume 16 (Roman numeral XVI) on page 29 of a publication abbreviated JAFL.
Use the tabs on the left hand side of this page to locate JAFL or the title of a books given as a reference. They go to an online version of the Bibliography and Abbreviations entries in the Motif-Index, For example, if you search the H - M tab for JAFL you will find *JAFL = Journal of American Folk-Lore. Boston, etc., 1888 ff.. Entries which are available in the HathiTrust or the Internet Archive are linked. Once connected to the text, use the page number given in the entry to navigate to the start page of the tale.
Story Search is an online database of "Motifs" from the Stith Thompson Motif Index of Folk-Literature. It can only search single words, but it might be useful in finding a motif number for a story. You can then search the Motif-Index to find the story.