The scores utilize an eclectic variety of musical styles, many culled from the Late Romantic idiom of Richard Strauss and his contemporaries that itself was incorporated into the Golden Age Hollywood scores of Erich Korngold and Max Steiner. While several obvious nods to Gustav Holst, William Walton and Igor Stravinsky exist in the score to Episode IV, Williams relied less and less on classical references in the latter five scores, incorporating more strains of modernist orchestral writing with each progressive score. The reasons for Williams' tapping of a familiar Romantic idiom are known to involve Lucas' desire to ground the otherwise strange and fantastic setting in well-known, audience-accessible music. Indeed, Lucas maintains much of the trilogy's success relies not on advanced visual effects, but on the simple, direct emotional appeal of its plot, characters and, importantly, music.
Star Wars often is credited as heralding the beginning of a revival of grand symphonic scores in the late 1970s. One technique in particular is an influence: Williams's revival of a technique called leitmotif, which is most famously associated with the operas of Richard Wagner and, in film scores, with Steiner. A leitmotif is a phrase or melodic cell that signifies a character, place, plot element, mood, idea, relationship or other specific part of the film. It commonly is used in modern film scoring as a device for mentally anchoring certain parts of a film to the soundtrack. Of chief importance for a leitmotif is that it must be strong enough for a listener to latch onto while being flexible enough to undergo variation and development.
Source: Wikipedia
