Most people know that the United States is made up of 50 states, but fewer realize the nation also has 14 territories—and arguably even more. Territories are areas that belong to and are governed by the "parent" country (though the territories may possess a high degree of autonomy) but that have not been granted statehood. The United States has overseen territories since 1789, and 31 of those territories have gone on to become U.S. states, including Alaska, Hawaii, Tennessee, Illinois, Ohio, Indiana, Michigan, and Wisconsin. While only five of the 14 current U.S. territories are permanently inhabited—American Samoa, Guam, Northern Mariana Islands, Puerto Rico, and the U.S. Virgin Islands—there exists a growing sentiment that most populous of these, Puerto Rico, should be given the option of becoming America's 51st state.
As mentioned, Puerto Rico is the most populous U.S. territory, home to more than 3.3 million people. It was acquired by the U.S. in 1899, along with Guam. Each of the five inhabited territories (marked with * above) have their own governments and are largely autonomous, though they are ultimately still beholden to the U.S. government. However, territories differ from states, which share sovereignty with the federal government, in that they do not share sovereignty. The United States federal government is allowed to explore, exploit, conserve, and manage all of its territories.
Four of the five populated territories (all but the U.S. Virgin Islands and American Samoa) are "organized" under the Organic Act. The citizens of organized territories are considered U.S. citizens, but they cannot vote in federal elections (although they can vote in primaries). Citizens of American Samoa, by comparison, are considered U.S. nationals (unless a parent is a citizen) and must apply for full citizenship.
In addition to the 14 existing U.S. territories, there are a handful of former territories worthy of mention.
The United States currently has 14 territories, with Puerto Rico being the most populous with 3.3 million people.