The US immigration visa is a stamp, which you need to have to be legally eligible to travel in the US. This stamp is an approval from the US State Department and from the US Department of Homeland Security. However, it is very important to keep in mind that just because you have a Visa, it does not guarantee that you will be able to enter this country. Considering the growing number of terror attacks, the laws have become very complex in this regard. You may be denied an entry on the basis of an array of other reasons even if you have a valid visa. The type of visa you need depends upon your purpose of visiting the United States of America.