US law divides employment-based residencies by three main categories (known as preferences)
First Preference Goes To...
Persons of extraordinary ability in the sciences, arts, education, business, or athletics. To qualify for this you need documents showing sustained national or international acclaim and recognition in the field of expertise. If you qualify you don't need a job offer as long as you're entering to continue work in the field in which you have extraordinary ability.
Outstanding professors and researchers with at least three years experience in teaching or research, who are recognized internationally. You do not need a labor certificate for this category either, but you must have a job offer.
Certain executives and managers who have been employed at least one of the three preceding years by the overseas affiliate, parent, subsidiary, or branch of the U.S. employer. You must be coming to the U.S. to work in a managerial or executive capacity. You do not need a labor certificate for this category but you must have a job offer.
Second Preference Goes To...
Professionals Holding Advanced Degrees, or Persons of Exceptional Ability in the Arts, Sciences, or Business. For this category, you must have a labor certification or Schedule A designation or establish that you qualify for one of the shortage occupations in the Labor Market Information Pilot Program. This category also requires a job offer. You can apply for an exemption from the job offer and labor certification if the exemption would be in the national interest.
Professionals holding an advanced degree are defined as... someone who has higher than a bachelor's degree, or a bachelor's degree and at least five years of progressive experience in the profession.
Persons with exceptional ability in the arts, sciences, or business are defined as... Someone who has a degree of expertise significantly above that ordinarily encountered within the field.
Third Preference Goes To:
Skilled Workers, Professionals Holding Bachelor Degrees and Other Workers This requires a labor certification, or Schedule A designation, or evidence that they qualify for one of the shortage occupations in the Labor Market Information Pilot Program.
Skilled workers are defined as as...people capable of performing a job requiring at least two years of training or experience