Our immigration system has three main “hooks” for those who want to come to the United States permanently: family, employment, and humanitarian grounds.
"Family" refers primarily to people who are beneficiaries of an approved relative petition (filed by a someone who is a U.S. citizen, a lawful permanent resident, or holder of one of several other specific types of visa). For instance, a U.S. citizen might file a family petition for her parents who are still living abroad.
In some cases, an individual may also apply to remain permanently in the United States based on the terms of their specific employment visa. This typically includes those living and working in the U.S. with certain high-skilled visas, such as the H-1B.
Finally, many people immigrate to the U.S. seeking humanitarian relief. That may include asylum/refugee status for survivors of persecution abroad; the “T Visa” for survivors of human trafficking; and the “U visa” for survivors of crimes committed and reported in the U.S., among several others.
In addition to these three broad avenues for immigration, thousands of people come to the U.S. each year as “nonimmigrants."