An immigrant visa provides permanent legal status in the United States, through what has been known as a green card. Permanent legal status is generally available through family or employment based visas.
To obtain a family-based visa, you must be sponsored by a U.S. citizen or by someone with permanent legal status in the United States. A person with permanent legal status only may sponsor a spouse or child, but no other family members. Before a family-based visa will be granted, you must demonstrate an ability to support the person seeking a visa.
There is also a priority for entry: unmarried adult children, then spouses and their unmarried minor or adult children, then married children, and then siblings.
The immigration laws give preference to workers in the following order: priority workers, professionals with advanced degrees or persons with exceptional skills, and skilled, professional or other workers, and special immigrants. Most employment-based visas require that you get a labor certification before the visa will be approved.