A visa is a type of permit that gives a non-citizen permission to enter a country. In the United States, there are dozens of types of visas, each with a specific application process, rules and requirements.

U.S. visas can generally be broken down into two broad categories:

• Immigrant: Meaning the person wants to permanently live and work in the country
• Non-immigrant: Meaning the person wants to visit the United States, but eventually return to his or her home country.

Within both the immigrant and the non-immigrant categories, there are many more subdivisions. For example, non-immigrant visas include visas for tourists, visas for students and visas for temporary workers. For immigrants, there are visas for immediate family and employer-sponsored visas, among others.

"One main purpose of the US immigration system and applicable non-immigrant and immigrant visa options is to be able to have some control over how many foreign nationals may be approved for various activities each year and what those foreign nationals are allowed to do in the US," says immigration lawyer Andrew Wilson, a partner at Serotte Reich Wilson, LLP.

Types of Non-Immigration Visas

Citizens of 35 countries who are visiting the United States for work or pleasure, and planning to stay for less than 90 days, may be eligible to enter the country under the Visa Waiver Program. The program—for citizens of countries such as the United Kingdom, Australia and Japan—allow people to come to the United States without a visa.

Prior to entering the US under the Visa Waiver Program, visitors must get approval through theElectronic System for Travel Authorization, or ESTA. Visitors must also have a valid passport.

Visitors who are ineligible for ESTA will have to obtain a visa prior to arriving in the United States. The most common types of visitor visas are:

• B-1 Business Visa: For visitors to the country who are attending meetings, seminars, negotiations or other professional business, but not seeking employment in the United States.
• B-2 Tourist Visa: For visitors to the country for tourism or medical treatment.
• K-1 Fianc?(e) Visa: Allows US citizens to bring their foreign fianc?(e) to the United States for the purpose of getting married. (After the wedding, the foreign spouse will have to apply for conditional permanent residence status.)
• Student Visas: There are a couple types of student visas, which allow non-citizens to get an education in the United States.

Types of Immigrant Visas

There are a number of types of immigrant visas. Among the most common:

• V-1, V-2 and V-3 Family-Based Visas: These visas allow green-card holders to bring their spouse and children to the United States
• IR-3 and IR-4 Visas for Adopting Foreign Orphans: These visas allow US citizens to adopt a child from a country that allows foreign adoptions.
• E-1, E-2, E-3, E-4 and E-5 Employment-Based Visas: These visas allow various skilled and unskilled workers to permanently relocate to the United States

Obtaining a Visa

Before trying to enter the United States, foreign citizens should visit US Citizenship and Immigration Services website to learn more about the visa(s) they might be eligible for, the visa application requirements and any limitations or restrictions they might encounter. Your local US embassy or consulate may also be able to answer questions.

People seeking immigrant visas, as well as foreign visitors who have encountered problems trying to enter the United States, may want to seek out the advice of a US immigration attorney.

"Immigration law is very complex," Wilson says. "Many processes entail important procedural and compliance steps. One wrong move can delay case or result in a denial. There are some processes that individuals can handle on their own if they are comfortable, but the majority normally require attorney representation."

Jennifer E. King co-authors the Lawyers.com blog.