Just like any other passport, a United States passport is a document, normally in the form of a small book that is issued to citizens and non-citizen nationals of the United States of America.
Just like a US visa, a US passport is issued exclusively by the U.S. Department of State. However, though similar in purpose with a US visa application, a US passport is the opposite of a USA visa as it is issued to US citizens and non-citizen US nationals for the purpose of international travel.
This means that as a United States visa enables a foreign national to travel to the United States, a US passport on the other hand enables US citizens and non-citizen nationals of the United States of America to travel outside of the borders of their home country.
U.S. passport booklets are valid for travel by Americans anywhere in the world without obtaining any further documentation (such as visas) prior to taking the trip, although a US passport does not automatically grant the holder the right to enter any country, in the similar way as an approved USA visa application.
Aside from being an essential international travel document, a US passport also functions as proof of citizenship, certifying the identity and nationality of its holder. The elements of identity showcased in the passport are name, date of birth, sex, and place of birth. So a person holding a U.S. Passport is internationally recognized as a US citizen.
In cases when visas are required, the visa stamps or stickers are also placed on the pages of the US passport. These pages of the passport are also where border control officials will also stamp the entry and exit stamps upon entering or exiting a country.
So, having a US passport is very essential especially to those who engage in international travel not only as a means to gain entry to another country but also as a proof of identity and nationality.
In accordance with US Federal law, all US citizens are required to enter and exit the United States on a valid US passport, even if they are also citizens of another country. So, you must make sure that your US passport is valid as United States citizens are not allowed entry back to the United States if their passport is expired, along with naturalization certificate, birth certificate, or a passport issued by another country.
Also, contrary to what airlines may say, there is no letter the embassy can provide nor is there a way to extend the validity of an expired passport that will enable travel by citizens who do not have US passports.
So, if a US citizen is abroad and he does not have a valid passport, he or she is required to have their US passport renewed at a US Embassy or Consular Agency before he can travel home.
This application for a United States passport made abroad is then forwarded by the U.S. embassy or consulate to Passport Services, a unit of the Bureau of Consular Affairs within the Department of State where the responsibility for passport issuance lies, for processing in the United States. The resulting passport is then sent to the embassy or consulate for issuance to the applicant.