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A film score is the music composed specifically for the movie to serve and enhance the story. It is usually instrumental only so as not to compete with dialog. It is also called underscore (a term I, as a composer, find ever so slightly belittling) or even incidental music (that one makes me bristle a bit).

The term soundtrack can refer to several things, depending on use. In the broadest sense it simply means everything you hear in the movie - sound effects, dialog, music. This meaning is usually applied in a more technical setting, in reference to the sound as opposed to the visuals. Soundtrack can also refer to the entirety of the music in the film, encompassing score AND songs that were either licensed or written for the film. Confusingly enough, oftentimes record companies will release a soundtrack from a movie which does not include any of the score. Generally that's because people are more interested in listening to songs from the movie that were written to stand alone, as opposed to score which was written to be part of the whole of the film. So the term soundtrack can also refer to only the songs in a movie, excluding the score.