A soundtrack is usually lyrical music (but not all the time) that was either written for a film by a band, but not an ORCHESTRA, or pre-existing songs that the director believes fits a certain scene or sequence well.
A score is dramatic music written exclusively for a film, usually performed by an orchestra and written by one or more composers that were hired for the film. For example, the new Friday the 13th movie score is by Steve Jablonsky, which is the music that appears only in that film alone; but you hear other songs during the movie like "Sister Christian" by Night Ranger that existed well before the film was shot. That's on the soundtrack release that came out this week which is NOT original music from the film. However, ON the soundtrack, the last track is "Friday the 13th Main Titles" by Steve Jablonsky. THAT is from the actual score.
Example: Indiana Jones and the Kingdom of the Crystal Skull
Score: music for the film by John Williams, such as the music that plays during the jungle chase sequence in the film, or when the UFO is taking off at the end.
Soundtrack: "Ain't Nothin but a Hound Dog" (not sure if that's the actual song title) which plays during the opening sequence. It's a pre-existing song NOT written by the film's composer, John Williams.
The score and soundtrack are two different entities.