This is a basic overview of the five main ways to get a greencard. Each of these five are broken down into many subcategories.
Family. U.S. Citizens and Lawful Permanent Residents (LPR) can petition for certain family members such as spouses, children, parents and siblings. There are two subcategories, one for immediate relatives (the fastest) and one for preference petitions (subject to the monthly published visa bulletin).
Employment. An individual can also obtain permanent residency through employment. The speed of these cases depends on the qualifications of the individual and the type of job offer. The fastest is for persons of extraordinary ability, then persons of exceptional ability and those holding advanced degrees, then skilled workers and professional and then unskilled (other) workers.
Asylum. If a person has a well founded fear of persecution based either on gheir race, religion, nationality, political opinion or membership in a particular social group, then they can qualify for asylum protection in the United States. If granted, that individual can apply for a green card after one year.
Investment. If an individual can invest $1 Million in a commercial enterprise that creates 10 jobs, they can obtain permanent residency. Alternatively they may invest approximately half that amount in a Regional Center or $500,000 in a Targeted Employment Area, that is one with high unemployment or in a rural area.
Diversity Lottery Visa. Each year 55,000 greencards are subject to a lottery for individuals from countries with low immigraiton rates to the United States. As the name suggests, its a lottery.