Puerto Rico is part of the United States. Though it is not a U.S. state, it is a Commonwealth of the United States, and Puerto Ricans are U.S. citizens.
Puerto Rico's currency is the U.S. dollar, it uses the U.S. Postal Service, Puerto Ricans serve in the U.S. armed forces and the U.S. flag flies over its capital. However, Puerto Rico is somewhat autonomous in that the people elect their own public officials and the day-to-day governing is mostly in their hands. Also interesting to note, Puerto Rico competes separately from the U.S. in both the Olympics and the Miss Universe pageant. Puerto Ricans do not file federal income tax returns unless they work for the federal government.