Earth is the element of substance. The citizens of the Earth Kingdom are proud, persistent and enduring. They strive to live in harmony with the other nations.
Fire is the element of power. The people of the Fire Nation strive for success, and use aggression and offense in order to be victorious.
Air is the element of freedom. The Air Nomads continually sought spiritual enlightenment, and freedom from worldly concers, which resulted in them being all airbenders.
Water is the element of change. Waterbenders deal with the flow of energy — they let their defense become offense, turning their opponents own forces against them. However, according to the film, water is the element of acceptance.