The United States has a fairly generous policy in admitting foreigners to the country as refugees, harkening back to the 1950s when several laws were passed to provide for people who escaped communist regimes.
In 1980, Congress expanded the immigration law beyond those fleeing communist countries to welcome anyone who had to leave their own country because they had been persecuted or feared being persecuted based on race, religion, nationality, political opinion, or membership in a social group. The law was intended to align with United Nations’ conventions on refugees, developed in 1951.
That appears to be how the two brothers suspected of bombing the Boston Marathon came in to the United States. They came as minors from the troubled area near Russia's Chechnya region. It is not clear whether the family came to the country as refugees, in which they apply for visas before they enter, or as asylum-seekers, who can apply for legal status once they get here.
In either case, unmarried children under 21 can be admitted at the same time as their parents or within two years after their parents arrive. They are considered “derivative” refugees of the primary applicant and must prove their familial relationship.
Refugees must be referred to the Department of Homeland Security through a refugee admissions program. Their applications are decided on a case-by-case basis in a “non-adversarial” interview with an immigration officer, according to DHS. The screenings are mainly designed to ensure that the applicant is eligible to enter the United States (i.e., has committed no crimes) and has not established residency in another country.
All refugees get work visas immediately upon arrival and can apply for green cards within one year. If they wish to travel abroad, they must procure a refugee travel document from the Department of Homeland Security in order to re-enter the United States.
Asylum seekers must wait 150 days after applying for asylum to get work permits. They can apply for green cards one year after being granted asylum.
The numbers of immigrants coming to the country under such conditions is relatively small compared with general migratory patterns, which equate to roughly half a million people a year. In 2010, former Immigration and Naturalization Services Commissioner Doris Meissner said the refugee program since 1980 had allowed 3 million people to enter the United States.