An immigrant is a foreign national who has been authorized to live and work permanently in the United States. If you want to become an immigrant based on a permanent employment opportunity in the United States, or if you are an employer that wants to sponsor someone for lawful permanent residency based on permanent employment in the United States, you must go through a multi-step process.

First, foreign nationals and employers must determine if the foreign national is eligible for lawful permanent residency under one of categories to lawful permanent residency.
Second, most employment categories require that the U.S. employer complete a labor certification processing for the applicant, and submit it to the Department of Labor. Labor must either grant or deny the certification request. Qualified alien physicians who will practice medicine in an area of the United States which has been certified as underserved by the U.S. Department of Health and Human Services are relieved from this requirement. Qualified aliens who meet the requirements as extraordinary ability, outstanding professor/researchers and national interest waiver are also relieved from this requirement.
Third, INS must approve an immigrant visa petition, for the person wishing to immigrate to the United States. The employer wishing to bring the applicant to the United States to work permanently files this petition. However, if a Department of Labor certification is needed, the application can only be filed after the certification is granted. The employer acts as the sponsor (or petitioner) for the applicant (or beneficiary) who wants to live and work on a permanent basis in the United States.