The United States is a republic, which means that the people hold the power and they elect representatives to decide what to do with it.
It is also a federal country, meaning that powers are divided between central government and those in the individual states.
Although these powers have changed over time, the US constitution originally only gave certain powers to the federal government. Many government services are still the responsibility of state governments.
Each state elects a governor and politicians to sit in the Senate and House of Representatives – the two chambers of the legislature.
Federal power is divided into three separate branches of government – the president and his cabinet (the Executive), the two chambers of the US Congress (the Legislature) and the courts (Judiciary).
The United States holds elections every two years. Once every four years there is a presidential election, with congressional elections held at the same time.
Mid-term elections take place in-between presidential elections, in the middle of the presidential term.
Elections for the House of Representatives are held every two years. Senators have six-year terms, with one-third elected every two years.
State governors serve four-year terms with about half up for election every two years.
Voters also get to choose their party’s candidate in the main election. Voters register which party they support, and can then participate in primary elections.
Before he became the Republican candidate for president in 2000, George W Bush had to beat Republican Senator John McCain in primary elections.
Primaries are sometimes quite close to the general election, and once they are over, candidates must turn quickly to the general election.
America declared its independence from Britain in 1776.
The Declaration of Independence read: “We hold these truths to be self-evident: That all men are created equal, that they are endowed by their Creator with certain unalienable rights; that among these are life, liberty, and the pursuit of happiness.”
Eleven years later, the separate states joined together to form the United States. At Philadelphia in 1787, a convention attended by the most important men of the day produced a constitution.
Unlike the UK, the US has a written constitution. It has four important principles: republicanism, federalism, the ‘separation of powers’, and ‘checks and balances’.
The president’s main function is to enforce the law, by running the federal government. He also proposes laws and the annual budget to Congress, is commander-in-chief of the US armed forces, and is broadly responsible for foreign policy.
He can also veto bills passed in the Congress, although that veto can be overridden if two-thirds of the Congress vote against it.
The president is elected every four years, but cannot serve more than two terms.
His cabinet members are not elected and are not normally members of Congress. They have to leave Congress to join the cabinet.
The Congress is responsible for passing laws and the budget. It has two chambers – the Senate and the House of Representatives.
The two chambers share the power to declare war, while the Senate must approve treaties agreed to by the president. Senators must also approve the appointment of judges and many government officials.
Every state has two senators, but the number of members of the House depends on the population.
For example, a state like Wyoming with a small population has two senators and just one member of the House, while a state like California with a huge population still has two senators, but 43 members of the House.
In total there are 100 Senators and 435 members of the House of Representatives.
The third part of government is the courts system – known as the Judiciary. The Supreme Court has nine judges who decide whether laws passed are in line with the constitution.
The judges are nominated by the president, and have to be approved by the Senate. They are extremely powerful positions. The judges are appointed for life, so a vacancy can only arise through the death, resignation, retirement, or impeachment of a sitting judge.
The US has a two-party system, which makes it very hard for anyone who is not a Democrat or a Republican to be elected. Almost everyone in Congress is a Democrat or Republican.