Overview of Discretionary Reprieves from Removal: Deferred Action, DACA, TPS, and Others [April 10, 2018] [open pdf - 996KB]
"The Immigration and Nationality Act (INA) establishes a system of rules as to which non-U.S. nationals (aliens) may enter the United States and under what conditions. It sets forth, for instance, three primary categories--family-based, employment-based, and diversity-based-- through which an alien may qualify for an immigrant visa and thereby seek admission to the United States as a lawful permanent resident (LPR). The INA also establishes requirements for the admission of refugees, and delineates the categories of aliens who may be admitted temporarily as nonimmigrants for particular purposes such as study, tourism, or temporary work."
CRS Report for Congress, R45158