Deportation, also known as "removal" in legal terms, occurs when the federal government orders that an illegal immigrant be removed from the United States. Deportation occurs for various reasons, but usually happens after a violation of criminal or immigration laws.
Visa holders need to be ready to prove that they are entering the United States for permissible reasons and that they intend to leave in accordance with the terms of their visa. In situations of inspections at the United States border, however, foreign nationals can sometimes be determined “inadmissible” and will not be allowed entry into the United States. Those people are normally either allowed to withdraw their applications for admission into the United States or are simply denied entry into the United States and must leave immediately.