There is no single agreement that affords copyright protections around the world. The United States has copyright relations in many other countries, but you don't automatically have all of the same rights to protect your work as you do in this country. At the same time, be aware that, just because an item was published in another country, you cannot just use it as though it had no copyright. 

You can get basic information and learn how to protect your copyright from the site Stop Fakes.

The government's copyright site includes this page on copyright in other countries, including treaties and agreements in which we participate.