A film score (also sometimes called background score, background music, movie soundtrack, film music or incidental music) is original music written specifically to accompany a film. [source]
In 1949, Aaron Copland outlined five categories of film music function:
In animation and film, "Mickey Mousing" (synchronized, mirrored, or parallel scoring) is a film technique that syncs the accompanying music with the actions on screen. [source]
A leitmotif is a recurring musical idea (a melody, chord sequence, rhythm or a combination of these) which is associated with a particular idea, character or place. For example; whenever Darth Vader appears in Star Wars the same tune is played.