American government is composed of three branches: the Judicial, which oversees the legal system; the nonpartisan Legislative, which is responsible for writing laws; and the Executive, which is led by the President and his or her staff. Together, the system provides a set of “checks and balances” in order to ensure that no one branch or political party becomes too powerful and influential.
The two most dominant American political parties of the last 70 years have been the Democrats and Republicans. Democrats are left-leaning, in favor of more government activity and stronger social safety nets. Republicans are their right-leaning foil, in favor of smaller government and lower taxes. (The two parties’ political leanings were somewhat inverted in the 1950s and 1960s, a complicated transformation that had to do with the Civil Rights Movement. You can read more about that here.)
Any American citizen over the age of 18 is eligible to vote, and national elections happen every two years for Congresspeople, four years for Presidents, and six years for Senators. Voting day for national elections is the first Tuesday after November 1st, and voter participation rates hover between 40% and 60%.