Chanson is the French word for "song". The word is often used in music to mean any song with French words, but it is more often used when talking about songs in which lyrics have been set to music by French classical composers.
Very often "chansons" refers to the French songs that were sung in the late Middle Ages and Renaissance. People who sang these chansons were called "chansonniers". They had various forms, including ballade, rondeau and virelai. Some composers at the time liked to set popular poetry to music. The earliest chansons were for two, three or four voices, many of them being for three voices. By the 16th century most were for four voices. Sometimes, the singers were accompanied by instruments.