To enter into the United States, foreign nationals must obtain immigrant or non-immigrant visas, based on the purpose of their travel and immigrant visas are meant for the people who wish to permanently live in the United States. Foreign nationals can also obtain non-immigrant US visas and travel to the United States but these temporary visas will only permit them to travel to the country as non-immigrants and stay in the United States for a particular time period but permanent immigrant visas will permit the foreign nationals to travel to America as permanent residents.
You will become eligible for an immigrant visa, if you are sponsored by your relative who is a US citizen. However, you need to be a US citizen’s immediate relative or must belong to one of the preference categories as US citizens can sponsor their parents, siblings, spouses and children but not their grand parents, nephews, nieces, uncles or aunties. Likewise, if you manage to get a full time job in the United States and if the employer who is offering you that job is willing to sponsor you for a US Green Card, you may become eligible for an immigrant visa. Similarly, the United States conducts a lottery program that randomly selects winners and issues immigrant visas to the eligible winners of the lottery program. If you take part in the Green Card lottery program and win, you will become eligible for a diversity immigrant visa, with which you can travel to the United States as a permanent resident.
Immigrant visas are issued by the consular offices located in foreign countries. To sponsor a family member, a US citizen must file an immigrant petition, Form I-130, for the family member and a US employer to sponsor a foreign worker, must file Form I-140, for the worker. After these immigrant petitions are approved by the USCIS, these petitions will be forwarded to the consular offices in the countries where the beneficiaries reside. After approving those petitions, the consular officers will inform the beneficiaries about the approval of their petitions and after that, the beneficiaries must file petitions for immigrant visas. After getting permanent visas, the beneficiaries can travel to America as lawful permanent residents. USCIS will mail their Green Cards to them after they get into the country as lawful residents.
Non-immigrants in America who are sponsored by their US citizen relatives or US employers, while in America, may not be required to obtain immigrant visas as they can just file petitions for adjustment of status after the USCIS approves the immigrant petitions filed by their sponsors and become lawful residents.