Economic classes are generally divided into the rich upper class, the middle class, and the poor lower class. People in the upper class are so wealthy that they don't have to work. The middle class work for a living midway in comfort between wealth and poverty. The lower class are so poor that they often beg for alms.
The conditions of the three classes has varied by time and place. In free societies, upward mobility makes it possible for poor people to become middle class, and this is rather common. Less frequently, middle class people become rich.
In Communist countries, as in other highly stratified societies, people rarely change class. This is ironic, because Communists justify the revolutionary overthrow of "Capitalism" by saying they will create a classless society, but actually they just replace the old upper class of with a new one; see Nomenklatura.