that were used during a movie
, primarily as background, but sometimes more prominently as in the pre-remastered version of Heavy Metal
Soundtracks usually follow a certain theme, depending on the movie in which they are used. The one for Pulp Fiction contains mostly excellent selections from the '50's, while the one for The Matrix is primarily hard rock and industrial.
While all movies will have a score, not all movies have soundtracks. The soundtrack is made up of songs used in a movie, while the score is a recording of the musical themes used. John Williams is an excellent example of a score composer, and Danny Elfman writes both scores and soundtracks.