Short answers, pulled from the story.
A leitmotif is a short, recurring musical phrase associated with a particular person, place, or idea. It can take the form of a melody, a chord progression, or even a simple rhythm, and it is designed to retain its identity even when modified in rhythm, harmony, or orchestration on later appearances.
Richard Wagner is the composer most specifically associated with the leitmotif. His four-opera cycle Der Ring des Nibelungen, written between 1853 and 1869, uses hundreds of leitmotifs tied to specific characters, objects, and situations.
Wagner did not coin or endorse the word. His preferred term was Hauptmotiv, which he first used in 1877. The word Leitmotiv gained currency through the critic Hans von Wolzogen, who published a guide to the Ring in 1876. When Wagner himself used the word, he preceded it with "so-called".
The use of short, recurring motifs in orchestral music traces back to at least the early seventeenth century, including Monteverdi's L'Orfeo. French opera composers Gluck, Grétry, and Méhul developed "reminiscence motifs" in the late eighteenth century. The critic Friedrich Wilhelm Jähns first put the word leitmotif into print in 1871, describing the work of Carl Maria von Weber.
The 1931 film M is identified as one of the first films to use a leitmotif. The main character, a child serial killer, whistles "In the Hall of the Mountain King" by Edvard Grieg, and that melody becomes his musical signature throughout the film.
In the first Star Wars film in 1977, John Williams attached distinct themes to specific characters and concepts, including Darth Vader, the Death Star, and the Force. He expanded this system across the original trilogy, then returned to it for the prequel trilogy starting in 1999 and the sequel trilogy starting in 2015, adding new themes while incorporating the established ones.