The United States, as the name suggests, is a collection of independently governed states. At first, the state and federal governments were separate as in a layer cake. Over time, the state and federal governments have become more intertwined and cooperative to adjust to the needs of society.
The federal government, for example, is responsible for coining currency, distribution of the military and foreign relations. The federal government also is responsible for widespread social nets to help needy citizens, such as Social Security. Programs like it were put in place after the Great Depression as part of the New Deal in 1933.
The federal government also has the power to provide block grants or categorical grants to states. Block grants are sums of money for a general purpose like health or education. Categorical grants are for a very specific purpose previously determined and agreed to in order for the state to receive the money. Block grants provide more power to the states, whereas categorical grants provide the federal government oversight. Also, the federal legislative branch consists of the Senate and House of Representatives, that are each loyal and represent the interests of their respective state governments and can thus influence federal policy.