Art song, in the Western classical music tradition, can be broadly defined as a short piece, typically for solo voice with piano (though can be with other collaborative instrumental) accompaniment, of serious artistic intent.
Art song is distinct from other types of song, such as popular song, in a couple of notable ways:
1) the text is usually of high literary quality, and is set to music in a manner that reflects the correct declamation of it; and
2) the piano is given equal importance with the voice, and contributes significantly to the overall artistic effect, and even meaning, of the song.
Thus, the “art” of writing art song is in the sensitivity with which a composer responds to the imagery and feeling conveyed in the poetic text.
Historically, art song as a genre developed in the late 18th and early 19th centuries, and was shaped by social and economic factors. During this time, the burgeoning middle class became significant and highly influential consumers of music. The piano, with its dynamic and expressive qualities, became the favoured instrument to have in their homes. This led to a demand for music that could be performed in this context, and composers sought to meet it by writing vocal music that was accessible to this public.
Art song was also established within specific national traditions. Of these, the German lied and the French mélodie were the most significant, and the most influential on the development of art song in other countries, including in Canada. In the late 19th and early 20th centuries, many Canadian composers chose to study in Europe and as a result, they tended to write art song in either the German or French tradition, with the texts usually by German, British, or French poets.
Eventually, composers shifted their interest to Canadian poets and themes, both of which helped to shape a distinctly Canadian approach to the art song tradition, first by shifting their interest to Canadian poets and topics, and then later integrating Canadian folk material (from European and Indigenous sources) with modern methods of composition (e.g. polytonality, serialism, modal scales). Some composers created their own texts, while others incorporated new vocal techniques or used instrumental combinations for accompaniment.
Sources:
Chew, Geoffrey. “Song.” Oxford Music Online, 2001. https://doi.org/10.1093/gmo/9781561592630.article.50647
Dickinson, Peter, H. Wiley Hitchcock, and Keith E. Clifton. “Art song.” Oxford Music Online, 2001. https://doi.org/10.1093/gmo/9781561592630.article.A2240068
Hall, Frederick A., Lucien Poirier, and Helmut Kallmann. “Art Song.” In The Canadian Encyclopedia. Historica Canada. Article published 7 Feb 2006; last edited 20 Jan 2014. https://www.thecanadianencyclopedia.ca/en/article/art-song-emc