Nonimmigrant visas are for international travelers, (citizens of other countries), coming to the U.S. temporarily. This visa allows you to travel to a U.S. port-of-entry (airport, for example) and request permission of the Department of Homeland Security immigration inspector to enter the U.S. A visa does not guarantee entry into the United States.
International travelers come to the U.S. for a wide variety of reasons, including tourism, business, medical treatment and certain types of temporary work. The type of visa needed is defined by immigration law, and relates to the principal purpose of your travel.