Fire is element of power. The people of the Fire Nation have desire and will, and the energy and drive to achieve what they want. Earth is the element of substance. The people of the Earth Kingdom are diverse and strong. They are persistent and enduring. Air is the element of freedom. The Air Nomads detached themselves from wordy concern and found peace and freedom. Also they apparently had pretty good sense of humor! Water is the element of change. The people of Water Tribe are capable of adapting to many things. They have deep sense of community and love that hold them together through anything.
(Jenderal Iroh, Avatar the Last Airbender)