A film score is original music composed for a specific film. A soundtrack is a collection of pre-existing tracks licensed for the film. The distinction matters because they work differently.
A composed score can be precisely synchronized: Bernard Herrmann's strings in Psycho were written to match specific cuts. Licensed music arrives with prior associations: "Born to Be Wild" in Easy Rider carries biker-culture connotations Steppenwolf created independently.
The most sophisticated films use licensed music against its connotations deliberately. Kubrick's use of "Singin' in the Rain" in A Clockwork Orange makes violence cheerful by association — and destroys the song for the audience permanently.
Related: sound-design-diegetic, genre-as-contract