Welcome to America!
As defined in the 1951 United Nations Convention Relating to the Status of Refugees (the Refugee Convention), a refugee is defined as a person who: "owing to a well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a particular social group or political opinion, is outside the country of his nationality, and is unable to or, owing to such fear, is unwilling to avail himself of the protection of that country or return there because there is a fear of persecution..." The reasons for persecution are: race, religion, nationality, and/or membership of a particular social group or political opinion. A person who is persecution based on any other ground is not considered a refugee by the UN Convention.
How do refugees get to the United States? First, after fleeing their homeland due to war, ethnic cleansing or persecution, they cross national borders into a host Country.
"Fortitude" - A video from the Department of Homeland Security (DHS)
Secretary Jeh Johnson from DHS explains the process of screening Syrian refugees.