The required documents for each country depend entirely on the specific destination, and that country's relationship to the United States. Regulations for developed and Western European countries, for instance, differ greatly from the requirements of entering a country such as South Africa.
The green card, primarily dealing with immigration, is not needed for travel outside--or back into--the United States unless you are not a U.S. citizen. Visitors to the United States living for a specific time displayed on their green card must use the identification documents of their native country while traveling outside the U.S.
American citizens with an active U.S. passport can receive access to a wide variety of countries, including Mexico, Canada, Australia, and all Western and Eastern European countries. Though each country provides a different duration period for your stay, the common length of time you can remain in one country is three months without a visa.
If planning to visit South America, Russia, or areas of Asia and Africa, you will need to secure a short-term visa before leaving the country to be allowed travel abroad. For specific country requirements, including the length of time you are allowed to stay, visit the U.S. Department of State website (see the Resources link).
Every foreign country requires special documents for travelers intending to gain citizen status. For some countries, such as France, this requires a visa for each consecutive year you intend to stay, up until a certain point (usually two or three years), wherein you can apply for a green card or country-specific document.
