Immigration refers to the movement of people from one country to other. While the movement of people has existed throughout human history at various levels, modern immigration implies long-term, legal, permanent residence in that country. Short-term visitors and tourists are considered non-immigrants. Immigration across national borders in a way that violates the immigration laws of the destination country is termed illegal immigration.

Under this definition, an illegal immigrant is a foreigner who either illegally crossed an international political border, be it by land, water, or air, or a foreigner who legally entered a country but nevertheless overstays his/her visa in order to live and/or work therein.
Among all the countries, immigrating to the United States of America is consistently one of the most popular choices for overseas nationals wishing to make a new start, further their career, or join family members overseas. With huge cultural diversity and geographical variety, applying for immigration to the USA can provide a wealth of opportunities for potential US immigrants.
A citizen of a foreign country, wishing to enter the U.S., generally must first obtain a visa, either a non-immigrant 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.
A visa allows you to travel to the United States as far as the port of entry (airport or land border crossing) and ask the immigration officer to allow you to enter the country. Only the immigration officer has the authority to permit you to enter the United States. He or she decides how long you can stay for any particular visit. Immigration matters are the responsibility of the U.S. Department of Homeland Security.
There are two categories of U.S. visas: immigrant and non-immigrant.Immigrant visas are for people who intend to live permanently in the US. Non-immigrant visas are for people with permanent residence outside the U.S. but who wish to be in the U.S. on a temporary basis such as for tourism, medical treatment, business, temporary work or study.
Approximately 28.4 million foreign-born people live in the United States, representing 10.4 percent of the U.S. population.International visitors and immigrants add greatly to USA’s cultural, education and economic life, according to American Demographics, adding about $10 billion a year to America's economic output. More important is the contribution immigrants and their children make just by being there to provide workers and leaders for the future. If today's immigration totals hold steady, it will account for about two-thirds of U.S. population growth over the next 50 years.