A developed economy is one that has a strong exporting industry that generates income for the country. Some examples of developed economies are the United States, England, Canada, France. Usually they call developed economies of first world.
The most diverse scientific studies demonstrate the positive relation between economic development and standard of living. The more developed a country's economy is, the better the quality of life for its citizens. The opposite is true, the less developed a country's economy, the worse a country's social indicators are.