Legal immigrants are people who enter the country lawfully by obtaining a visa (immigrant and non-immigrant) from the American Embassy.\nThey usually go through background checks, criminal checks and have to provide medical records to be allowed to get the visa. They also have to wait for their priority date (each country has different priority dates). \nOnce they enter the U.S. they go to a Custom and Border Protection Offficer/Agent to be inspected before they are allowed into the U.S.\n\nIllegal Aliens bypass these checks by entering the country without inspection and any kind of checks.  Usually by crossing the border through Mexico or Canada.