A visa is an official stamp in your passport authorizing you to travel, work, or study in that country for a specified length of time. All countries, including the US, require a work permit visa for foreign workers; employers will not hire you without one.
For select countries, short-term work permit visas are available by application. In other cases, such as for long-term employment, foreign employers will need to apply for the visa on your behalf. Student work exchange programs can legally assist students in obtaining a short-term work permit visa. Unpaid internships and volunteer positions may or may not require a work permit, depending on the host country's regulations.
It’s always safe to assume that you’ll need a visa, especially if you’ll be traveling for a purpose other than tourism. Check out the visa requirements for specific countries by checking with the embassy or consulate of the country you will be visiting. Embassy and consulate contact information can be found on US Department of State’s page that lists websites of foreign embassies or the US Department of State’s Country Specific Information. Perry International’s Visa Requirement database can be useful in determining what type of visa, if any, is required based on the reason for travel and the traveler’s country of citizenship.