Programme music is descriptive, suggesting visual images or 'telling a story'. The descriptive idea or story-line is known as the programme.
Instrumental music that is free of a programme and exists purely for its own sake is known as absolute music.
Although descriptive music had always existed, orchestral programme music became very popular during the Romantic period (roughly the 19th century) when music developed close links with literature and painting.
Sometimes the programme was suggested by the title alone eg the Scottish and Italian symphonies by Mendelssohn where the title provides the source of inspiration rather than a storyline. At other times composers set out the programme in great detail, eg the Symphonie Fantastique by Berlioz.
Musical devices used to express the story or inspiration include: