The Congressional Research Service provided the following report on Immigration Consequences of Criminal Activity to the Congress in October 2006.

In general, aliens may legally enter the United States under one of three categories: (1) legal permanent residents (LPRs), who are also commonly referred to as immigrants; (2) nonimmigrants, who are aliens permitted to enter the United States temporarily for a specific purpose, such as for tourism, academic study, or temporary work; and (3) refugees, who are aliens facing persecution abroad and are of special humanitarian concern to the United States. There are two aspects for legal admission under each of these categories. First, an alien must fulfill the substantive requirements for admission under a specified category. For example, in order to enter the United States as a nonimmigrant student, an alien must demonstrate that he is a bona fide student at an approved school. Second, aliens who fulfill substantive requirements for admission may nevertheless be denied admission if they fall within a class of inadmissible aliens listed under Immigration & Nationality Act (INA) Section (§) 212. Once admitted, aliens remain subject to removal if they fall within a class of deportable aliens listed under INA § 237. The INA contains bars for admission and grounds for deportation based on criminal conduct.