According to the Wex Legal Dictionary, federalism is a governmental system in which two levels of administration control the same piece of land. Federalism is prominent in the governing structures of many current countries, including the United States, India, Belgium and Brazil.
The United States is a federal system because it has government at the state level that has its own responsibilities to the citizens and a government at the national level that has a different set of responsibilities to the same citizens. For example, states build roads, regulate corporations, govern land use and labor, and provide a number of other services for the citizens. The national government, on the other hand, regulates immigration law, provides currency, organizes the armed forces and conducts foreign policy. Sometimes, of course, which government is responsible for what comes into question, and disputes are resolved in the courts. Since the U.S. Civil War, the national government has generally been on the winning side in such disputes.
Other countries, such as Belgium, have a federal system because they are composed of a number of unique cultures that do not want to contend with a unitary national government. Belgium, for instance, includes three main regions: the French-speaking Wallonia, the Flemish-speaking Flanders and Brussels, the capital. Because these areas differ culturally and economically, the government federalized itself to give the different communities some autonomy. However, a national government still controls certain aspects of life.