A shed is a utility structure designed primarily for storage or workspace. A cabin is built with livable features in mind: bigger windows, more open floor plans, and a design that lends itself to spending extended time inside. An ADU, accessory dwelling unit, is a legal classification for a secondary residential structure with plumbing, electricity, and living amenities that meets local building codes for habitation.
The physical structures can overlap. You can finish a shed into a space that functions like a cabin. You can build a cabin that meets ADU requirements. The difference is mostly about intent, features, and what your local government classifies it as.
