Ballet is a very structured dance form, while modern dance is more free-flowing. Ballet dancers learn precise steps that must be done exactly right, with perfect form, while modern dancers are permitted to move with less precision. Ballet also relies on stage settings to help set the mood, much like in a play, while in modern dance it is entirely up to the dancer to portray the emotion behind the dance.
Ballet had its start in the 15th century and became a professional dance form in 1661, when King Louis XIV of France established the Academie Royale de Danse. Ballet is an old form of expression. Modern dance, on the other hand, came into being only in the early 20th century. It is a much newer form of dance. Ballet, as a result, is very set in tradition, whereas modern dance is freer to expand into new forms of expression.
Ballet's strict adherence to tradition and precision brought about the introduction of modern dance. Dancers who joined the modern dance movement were rebelling against ballet and its rigid rules and lack of room for personal expression. American dancers Martha Graham and Doris Humphrey were the first to break away from ballet's traditions and start choreographing their own, more expressive dance.
Ballet was developed out of the social dances of the 15th century and has not been influenced by other types of dance over the years. Modern dance, on the other hand, draws from tap, jazz and ethnic folk dancing as well as ballet. Modern dance choreographers are more free to explore other dance forms and integrate what moves them into their work, while ballet choreographers are bound to steps and positions that have centuries of tradition.