An international passport is used to travel abroad, although certain nations are excluded of this requirement (e.g. If you live in the US, you need an international passport to travel to for instance Europe, Australia and others). Simultaneously, US citizens are not required to have an international passport to travel to Canada or Puerto Rico (and certain other nations). If you reside in a country under the umbrella of the European Union, no international passport is required to travel to any of the other 26 EU countries, however you do need an international passport to travel to the US and and other countries.
If you are planning a trip to Europe, keep in mind that you will need your international US passport at European customs when entering Europe via the airport, and when going back Stateside.