The principles of separation of powers and checks and balances prevent any one section of the government from gaining too much power. In the United States, power is divided between the national, state, and local levels and is subdivided within each of these levels. For instance, the U.S. national government has three branches: Executive, Legislative, and Judicial; the Constitution gives each branch of government certain powers and does not allow them to exercise powers delegated to another branch.
The principle of checks and balances restrains the power of each branch of government by allowing each branch to "check" the powers of the others. In the United States, Congress (the Legislative branch) passes laws, but the President (Executive branch) can veto those laws and the Federal Courts (Judicial branch) can decide whether or not those laws are Constitutional. The President, in turn, appoints judges to the Federal Courts, and Congress can impeach the President or restrict his or her funding. These are just a few examples of how separation of powers and checks and balances operate in the United States.