US immigration laws have been set up to defend the United States from people who want to come into the country illegally for their own benefits. There are people who want to take advantage of the benefits and lifestyle many enjoy in the United States and are willing to do anything and everything to get into the country.
That's why US immigration laws were set up. They're assumed to make it possible for individuals who legitimately want to move to the United States to do so, and they're supposed to make it more difficult for those will negative, ulterior motives to get into the country. You can also visit http://www.amity-law.com/immigration-lawyer-los-angeles/ in order to learn more about the immigration laws.
Generally, a person who is not an American citizen but wants to enter the US needs to get a visa. This is according to US immigration laws set up to protect the country. The visa is a special document that's placed inside a passport. A passport can be considered to be a travel document issued by the traveller's country of citizenship.
The United States has relationships with certain countries so that according to US immigration laws visitors from those countries aren't required to get a visa.