Many Americans got their first, confusing glimpse of the country's complex immigration system when President Donald Trump announced he was temporarily banning immigrants from seven Muslim-majority countries from entering the U.S. and suspending the nation's refugee program.
Here's a look at the system:
WHAT IS A GREEN CARD?
A green card is a permit given to immigrants who are approved to be in the country indefinitely and become known as permanent residents.
Obtaining a green card can take decades and involves a meticulous process. Some people never receive one.
About a million people are granted permanent residency in the U.S. each year. A little less than half of them are sponsored by relatives who are American citizens, according to William Stock, president of the American Immigration Lawyers Association.
Stock says sponsors and applicants undergo interviews, criminal background checks, medical screenings and fingerprinting.
The process takes at least a year for the spouse of a U.S. citizen who has never been in the country. Immigrants with more distant relatives can wait years longer.
"If it's a sibling, it's 20 to 30 years for most countries," Stock said, adding that people from Mexico and the Philippines can wait 75 years for green cards.
WHAT IS REFUGEE STATUS?
Refugees from war-torn or dangerous countries must be diligently vetted before they can be resettled in the United States. The years-long process includes rigorous background checks for possible criminal and terrorist ties.
Refugees often live in camps outside their home country for years before the U.S. decides whether to accept them.
Only about 1 percent of refugees in the world are resettled in new countries, and the U.S. takes in about half of them, the State Department says.
The U.S. accepted about 85,000 refugees in the year ending in September.
Refugees differ from asylum seekers.
Refugees must be approved to move the United States. Asylum seekers arrive on their own and then seek protection by the U.S. government if they fear returning to their home country.
WHAT IS A NON-IMMIGRANT VISA?
Non-immigrant visas apply to people who come to the country on temporary status, including students, competitive athletes, tourists, human trafficking victims, au pairs and diplomats.
The category also includes foreign workers such as engineers and technical experts who work for Silicon Valley companies.
Some tech companies said they were dismayed by Trump's executive order because foreign employees represent a significant percentage of their workforces. Visas for such workers can usually be obtained within a year.
WHAT VISA IS USED FOR IRAQI INTERPRETERS WHO AID US TROOPS?
The visa program for Iraqi and Afghan translators and interpreters and others who worked for the American government was created in 2006.
Iraqis and Afghans who apply for the visas are vetted in a similar way as refugees and are also entitled to the same public and private resettlement services.
There is a tight limit on the number of such visas granted. The government granted 37,000 Iraqis and Afghans special visas from 2007 to 2015, according to the Congressional Research Service, a non-partisan group.
The service says government officials try to balance the need to issue the visas in a timely fashion to protect applicants with conducting appropriate security screenings to identify threats.