Short answers, pulled from the story.
A leitmotif is defined as a short musical idea that can be melodic, harmonic, or rhythmic and serves as a salient recurring figure within a composition. It represents the smallest structural unit with thematic identity and possesses special importance to bind an entire work together into a coherent whole.
The first printed use of the word leitmotif appeared in 1871 when critic Friedrich Wilhelm Jähns described Carl Maria von Weber's work using this terminology. Hector Berlioz employed a related concept known as idée fixe in his 1830 Symphonie fantastique prior to this publication date.
Richard Wagner stands as the earliest composer most specifically associated with the concept of leitmotif through his cycle of four operas titled Der Ring des Nibelungen between 1853 and 1869. He used hundreds of such motifs to relate specific characters, things, or situations throughout the massive work.
The movie M released in 1931 was one of the first films to use a leitmotif effectively by having a child serial killer whistle In the Hall of the Mountain King by Edvard Grieg throughout the film. This marked a transition where functions became simple announcements leading directly to cinema music.
John Williams composed the main shark theme for Jaws in 1975 using two alternating notes E and F. This musical choice created a recurring motif that remains one of the most recognizable themes in film history.