The music sounds in a minor key (tonality), often described as having a dull, dark, evil or sadder feel than major.
The minor scale is built on an order of tones and semitones and follows the musical alphabet (e.g. A B C D E F G A). The order is tone, semitone, tone, tone, semitone, tone, tone. This is the natural minor scale which sounds modal.
In order to make the scale sound better the last interval is reduced to a semitone. This makes the previous interval a tone and a half (e.g. A B C D E F G# A). This is the harmonic minor scale.
Hear an example of music in a minor key.