An autocracy is a system of government in which one individual holds all state power.
They are often characterized by long, uninterrupted reigns of power and are always characterized by an utter lack of checks and balances in government. The communist governments of Cuba, North Korea, and Venezuela can be considered to be run by autocrats. These leaders falsely claim to rule in the name of the people, but do not operate in a democracy and actually wield great power over individuals.
The United States has many checks to prevent autocracy, including democratic elections, Constitutional rule, and division of power between three branches of government. A president can pass executive orders, but the ability to pass legislation is divided among 100 Senators and 435 representatives, and judicial power is divided among 9 justices of the Supreme Court. An unpopular president can be voted out of office by the people, and as long as the Constitution is faithfully upheld state and local governments will still have their own authority within the state.