Millions of people enter and leave the United States every year. It is a popular tourism and immigration destination. However, if you are not a resident of the United States, you will most likely need a visa to enter the country.
A US visa is a stamp that you get on your travel document, more specifically your passport, which means that you are eligible to enter the United States. Having a US Visa does not necessarily mean that you will enter the US, as this is at the discretion of the Department of Homeland Security (DHS), and the Customs and Borders Protection (CPB) Inspectors.