A visa is permission to apply for entry to a foreign country. In most cases a visitor to the US must obtain a visa from a US consular office before departure from the home country. U.S. entry requirements are stricter than those of many other countries, partly because our internal controls on employment are much looser. Details of the applicable law and current regulations are available from the US State Department website: http://travel.state.gov/visa/visa_1750.html
Excerpts from: http://travel.state.gov/visa/visa_1750.html
About the Exchange (J) Visitor Visa
A citizen of a foreign country, who wishes to enter the United States, generally must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence. The type of visa you must have is defined by immigration law, and relates to the purpose of your travel.
The Immigration and Nationality Act (INA) provides the exchange visitor (J) nonimmigrant visa category for persons who are approved to participate in exchange visitor programs in the United States. This means that before you can apply at an American Embassy or Consulate for a J visa you must apply, meet the requirements, and be accepted for one of the Exchange Visitor Program categories through a designated sponsoring organization. If you are accepted as a participant in an exchange program, the sponsor will provide you with information and documents necessary to apply for the J visa to enter the United States.
Changes introduced shortly after September 11, 2001 involve extensive and ongoing review of visa issuing practices as they relate to our national security. Visa applications are now subject to a greater degree of scrutiny than in the past. Applicants affected by these procedures are informed of the need for additional screening at the time they submit their applications. So it is important to apply for your visa well in advance of your travel departure date.
A visa allows a foreign citizen to travel to the United States port-of entry, and request permission from the Department of Homeland Security, Customs and Border Protection, U.S. immigration inspector to enter the United States. A visa does not guarantee entry into the United States.
Student Applicants (for F-1 and M-1 visas)
If you are going to the U.S. primarily for tourism, but want to take a short course of study of less than 18 hours per week, you may be able to do so on a visitor visa. You should inquire at the appropriate U.S. Embassy or Consulate. If your course of study is more than 18 hours a week, you will need a student visa. Please read this information for general information on how to apply for an F1 or M1 student visa. For additional student related information, visit the EducationUSA website created by the Department of State, Bureau of Educational and Cultural Affairs to learn about educational opportunities for undergraduate and graduate study, opportunities for scholars, financial aid, testing, admissions, and much more.
In most countries, first time student visa applicants are required to appear for an in-person interview. However, each embassy and consulate sets its own interview policies and procedures regarding student visas. Students should consult Embassy web sites or call for specific application instructions.
Keep in mind that June, July, and August are the busiest months in most consular sections, and interview appointments are the most difficult to get during that period. Students need to plan ahead to avoid having to make repeat visits to the Embassy. To the extent possible, students should bring the documents suggested below, as well as any other documents that might help establish their ties to the local community.
Changes introduced shortly after September 11, 2001 involve extensive and ongoing review of visa issuing practices as they relate to our national security. It is important to apply for your visa well in advance of your travel departure date.
Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence. The visa allows a foreign citizen, to travel to the United States port-of entry and request permission of the U.S. immigration inspector to enter the U.S. The "visitor" visa is a nonimmigrant visa for persons desiring to enter the United States temporarily for business (B-1), for pleasure or medical treatment (B-2), or combination of both (B-1/B-2) purposes.
Business Visitor Visas (B-1) - For example, if the purpose for your planned travel is to consult with business associates, travel for a scientific, educational, professional or business convention, or conference on specific dates, settle an estate, or negotiate a contract, then a business visitor visa (B-1) would be the appropriate type of visa for your travel. After reviewing this website information, should you need additional information about business related (B-1) visitor visas, select Business Travel to the United States – What Type of U.S. Visa Will You Need.
Pleasure, Tourism, Medical Treatment - Visitor Visas (B-2) - As examples, if the purpose of your planned travel is recreational in nature, including tourism, amusement, visits with friends or relatives, rest, medical treatment, and activities of a fraternal, social, or service nature, then a visitor visa (B-2) would be the appropriate type of visa for your travel.
Persons planning to travel to the U.S. for a different purpose such as students, temporary workers, crewmen, journalists, etc., must apply for a different visa in the appropriate category.
NOTE: Representatives of the foreign press, radio, film, journalists or other information media, engaging in that vocation while in the U.S., require a nonimmigrant Media (I) visa and cannot travel to the U.S. using a visitor visa and cannot travel on the visa waiver program, seeking admission by the DHS immigration inspector, at the U.S. at the port of entry.
The Department of State recommended first source of visa information is this Visa Services internet site. Please review this information carefully. For an additional visa overview, select the Department of State visa portal site, Destination USA. As explained below, it is also recommended that you review the Embassy Consular internet site, for the country where you will be applying for your visa.