The immigrant visa [green card] gives the holder the following rights and obligations:
Live and work in the U.S.;
Apply to become an American citizen after holding a “green card” for a certain length of time;
Petition for a nonimmigrant visa for a spouse and unmarried children
Maintain the place of actual residence in the U.S [exception – commuters]
Pay U.S. taxes on a worldwide income
Not to remain outside the U.S. more than one year at a time without special permission
There are three main programs to obtain an immigrant visa:
First: Family Based Petition
U.S citizens may petition the Service to classify spouses, parents and unmarried children under the age of 21 as permanent resident aliens. Such immediate relative petitions are not controlled by an annual quota. Preference petitions may also be filed by permanent resident aliens on behalf of their spouses and children, but these petitions are subject to numerical quota and significant delays.
Second: Employment Based Petition
U.S. employers may file an immigrant preference petition on behalf of a foreign worker.
In order to qualify for a nonimmigrant visa [green card] through employment a foreign person must:
Have a specific job offer from a U.S. employer
Have the correct background in terms of education and work experience for the job been offered
There must be no qualified American Citizen willing or able to take the job-[Labor Certification] except in categories where Labor Certification is not required
Whether a foreign person needs a Labor Certification to obtain an immigrant visa [green card] through employment depends on the preference work category.
The five employment preference categories are:
Employment First Preference [No Labor Certification required]
Priority workers:  persons of extraordinary ability  outstanding professors and researches  transferring executive or managers of multinational companies
Employment Second Preference [Labor Certification required]
Workers with advanced degree or exceptional ability  advanced degree professionals  persons of exceptional ability
Note: A Labor Certification and job offer requirements could be waived in the second category if the applicant’s presence will ‘benefit’ the U.S. in the future. By ‘benefit’ the INS means a favorable impact on economic, employment, educational, housing, cultural or other important aspect of the country. The impact must be national and substantial.
Employment Third Preference [Labor Certification required]
Skilled or unskilled workers without advanced degree  professional workers (bachelor degree and less than five years of work experience)  skilled workers (at least two years of training experience)  unskilled workers (less than two years of training experience)
Religious workers and various miscellaneous categories of workers and other individuals  religious workers  foreign medical graduates  former overseas U.S. government workers  retired employees of international organizations  persons employed in the United States Consulate in Hong Kong  persons declared dependent on a Juvenile Court  accompanying spouses or children under 21 of a special immigrant Employment Fifth Preference [No Labor Certification required]
Individual investors willing to invest $1,000,000 in a U.S. business [or $500,000 in economically depressed area]
Third: Diversity Lottery Program
The U.S. also makes available a number of immigrant visas for persons who were born in certain countries and regions from which the U.S. has experienced relatively low immigration during the prior five years are the primary recipients of these so-called lottery visas. See: http://travel.state.gov/visa/immigrants/types/types_1318.html