Washington, the capital of the United States, is situated on the Potomac River in the District of Columbia. The district is a piece of land which doesn't belong to any state but to all the states.
Under the Constitution, the federal government is divided into three branches.The legislative power is vested in Congress and made up of two houses: the Senate and the House of Representatives. There are 435 members in the House of Representatives and 100 senators. Each state elects two members of the 100-member Senate.
The executive branch is headed by the President, who proposes bills to Congress, enforces federal laws, serves as commander-in-chief of the Armed Forces and with the approval of the Senate, makes treaties. President can veto a bill unless Congress by a two-thirds vote shall overrule him.
The vice President, elected from the same political party as the President, acts as chairman of the Senate, and in the event of the death of the President assumes the Presidency.The judicial branch is made up of Federal District Courts, 11 Federal Courts and the Supreme Court. Federal judges are appointed by the President for life. Federal courts decide cases involving federal law, conflicts between citizens of different states. The Supreme court may rule the law to be unconstitutional.
Now about the elections. The President is chosen in nation-wide elections every 4 years together with the Vice-President.
Constitution has been amended 26 times. The Bill of Rights guarantees individual liberties: freedom of speach, religion and so on. Later amendments abolished slavery, granted the right to vote to women and allowed citizens to vote at age 18.