A work visa is an official stamp in your passport authorizing you to work 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.