Simply, a chanson is the French equivalent of a “song”, usually of the simple, verse-repeating sort; that can be sung in performance, or with friends, or by one’s self, for the pleasure of it.
More technically the chanson was a form of song in various parts for voices, or for one voice with instrumental accompaniment, that developed in France and northern Italy during the 14th century, and flourished into the 17th century. It grew out of the music of the minstrels (the troubadours and trouvères), and like the ayre type of Madrigal, usually had the tune (sometimes a folk melody) in the tenor line. Josquin Desprez (1440-1520) is considered an expert.
During the 17th century it found itself increasingly influenced by the madrigal and, like the madrigal, its role was taken over by the cantata and the increasing importance of opera. As a song, though, a simple tune sung by “simple folk”, it remained, and does so today.
Chanson de Geste (song of deeds), a fore runner of the chanson, was a sung epic verse form, recounting some heroic story (deed) – The Song of Roland of about 1080 is the most well known example. The form vanished with the decline of the troubadours and trouvères in the 14th century, brought on by the devastation of the Albigensian Crusade and the lessening in importance of the chivalric ideal.
Chanson de toile' (work song) was a weaving or spinning song, the term current during the 11th and 14th centuries, usually telling the tale of the young, usually well-born, lover or wife, keeping herself busy whilst she awaits her lord’s return from war or crusade.
“Oxford Companion to Music”
“The Grove Concise Dictionary of Music”