Ais a composition for three, four, or five parts played or sung by voices or instruments; it begins with a presentation of a subject in imitation in each part and continues with modulating passages of free counterpoint and further appearances of the subject.

A fugue is a composition for three, four, or five parts played or sung by voices or instruments; it begins with a presentation of a subject in imitation in each part and continues with modulating passages of free counterpoint and further appearances of the subject.

A fugue is a type of musical form that is a composition or technique. A fugue is a type of way for writing pieces of music. It's basically a recipe for writing music.