The Immigration and Naturalization Act (INA) that governs the US immigration policy sets the yearly limit of worldwide permanent immigrants that can enter the US as 675,000, with certain exceptions.
The guiding principle behind Immigration policies of the US is to help their economy by admitting professionals with highly valuable, and in demand skills, promoting cultural diversity, protecting refuges, unification of families, etc. Accordingly, the broad categories of Immigration visas that US offers are:
Employment Based Immigration:
Every year the US permits 140,000 highly skilled professionals from across the world, to enter the US on a permanent employed based immigrant visa. These 140,000 visas are awarded to people with extra-ordinary skills with achievements in the areas of science, arts, sports, business, etc., applicants with advanced levels of degrees, those who have more than 2 years of professional training or experience, religious workers, businessmen who can invest up to $ 1 million and employs 10 full-time American employees.
If you have the right qualification and experience, a US employer may hire you and file application for a work permit paying the legal fee. After a few years, your employer may file your application for Green card, so you can permanently immigrate to the US.
Family Based Immigration:
If your spouse is a US citizen, or a Green card holder, if you are a minor child – an unmarried child of under 21 years old – of a US citizen, or parents of a US citizen, the US citizen has to be older than 21 years to file a petition to help his or her parents immigrate to the US. On an average, the US can accept 226,000 family based immigrants every year using the preference system, but the presence of many immediate family members in the US more than 480,000 people migrate to the US making use of family based immigration visa.
A US citizen or a legal permanent resident has to file the Family based immigration application for his relatives proving the relationship, the financial stability, and sign an undertaking that he would be financially responsible for his relatives once they enter the US as immigrants.
Refugee / Asylum Visa:
The US offers refugee or Asylum based immigration to those who face life-threatening conditions in their home country. People who face discrimination in their home country based on their race, membership in a social group, or religion can apply for a refugee visa from a transit country. The success of their application depends on the risks they face in their home country, the relationship of US with the group they are members of, and if they have any relatives in the US. Asylum is awarded to people who are already in the US. Refugees and asylees can become permanent residents after a year of their stay in the US. People can also immigrate to the US under the Diversity Immigration visa program.
You can apply for citizenship, once you complete five years of complete residency from the day you get green card, and demonstrate good character, pass English, and American history tests.