Introduction: "Congress has expressed interest in the visa categories by which foreign nationals may enter or be present in the United States. U.S. immigration policy is governed largely by the Immigration and Nationality Act (INA), which was first codified in 1952 and has been amended significantly several times since. The United States has long distinguished permanent immigration from temporary immigration. Permanent immigration occurs through family and employer-sponsored categories, the diversity immigrant visa lottery, and refugee and asylee admissions. Temporary immigration occurs through the admission of visitors for specific purposes and limited periods of time, and encompasses two dozen categories (which are commonly referred to by the letter and numeral that denote their subparagraph in the INA)."
CRS Report for Congress, R45938
Congressional Research Service: https://crsreports.congress.gov/