Centralism is the concentration of political power into central governing bodies that each govern a very large area of land, a very large amount of natural resources, a very large number of people, and/or a large variety of people, whereas decentralism is the diffusion of political power to smaller areas of land, smaller quantities of people, and/or fewer varieties of people. In the future, the world, and various countries, may move toward either centralism or decentralism.
The recent trends have gone in both directions. In the early 1990s, there was a movement toward decentralism, as the Soviet Union broke up, Czechoslovakia broke into 2 countries, and Yugoslavia broke into several countries. There has also been a recent move towards centralism in some places. In Russia, Vladimir Putin has centralized the governmental control of Russia, bringing it back closer to how it was in Soviet times. Old nationalist socialist elements in Europe, leftover from the World War 2 era, have gained power and established the European Union, which requires that all of it's member nations follow it's ideology. The European Union has thus finally succeeded in doing what Hitler failed to do.