The title says it all: What is the difference between a movie's soundtrack, and its score?
A soundtrack can often include complementary popular music and may not be original to the movie. Soundtracks can also offer a complete experience when played separately from the film and some songs from it may not even be present within the film at all!
A score, on the other hand, is usually non-diegetic music that compliments what is seen on screen. Film scores may be pleasant experiences when played separately from a movie, but they are usually intended to primarily support the movie. Original scores are often referred to as original sound tracks (OST), and will usually be released along-side the mainstream soundtrack.
For example, Guardians of the Galaxy has both a soundtrack and an original score. The original score is mostly instrumental music that was composed for the film; the soundtrack is a collection of popular music that influences the film or is featured within it.
The soundtrack can be one of two things.
From an editing perspective it is everything which is audible and can include :
- Original dialogue
- Additional (overdubbed) dialogue
- the score
- foley tracks (ie prost production physical sound effects) in practice this will often be most non dialogue sounds other than music
- sound special effects (explosions, monster noises etc)
- licensed songs or other audio
In terms of merchandising a 'soundtrack' album will usually be non-original music (ie not made especially for the production) and occasionally excerpts of dialogue. In some cases songs which only feature very briefly or even not at all may be included if they are deemed appropriate for either thematic or commercial reasons and may be significantly different versions from what appears on screen. this can include songs which are eg playing on the radio or live in a scene which characters can actually hear as opposed to mood music which, by convention characters are oblivious to.
A good example of this is Apocalypse Now where the soundtrack is mostly 60's pop and rock songs (and very famously a bit of Wagner) , most of which are actually heard and reacted to by the characters on screen. Similarly Pulp Fiction.
In contrast a score is usually written specifically for a film and will often be carefully matched to beats and cuts in the final edit. Often is is based around a unifying theme and intended to enhance the emotional experience for the audience. In many cases this will be written in an orchestral or symphonic style.
Often a score will play under quite a large fraction of the run-time of the film and often has distinct motifs which are associated with a particular character or location.
Score also apples to the sheet music used for musicals (as opposed to the script for the spoken dialogue), although this is obviously a bit of a different thing.
Some productions like LOTR and Blade Runner have scores which stand up in their own right and may be released in an edited form as an album. In other cases they may be more subtle mood music.