Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence.
Visa vs. Status. A U.S. visa is an official authorization affixed to a valid passport, granting permission to enter the U.S. in a particular non-immigrant classification. Only a U.S. Consulate can issue visas to enter the country. Consulates may have offices in a U.S. Embassy or they may have their own stand-alone offices; they are always located outside of the U.S. Therefore, you can only apply for a visa if you are outside of the U.S. At a U.S. port of entry (such as an airport) you must show your visa to an officer from the Department of Homeland Security. By presenting your visa, you are applying for the related I-94 status. Once in the U.S., you will not need a visa until the next time you wish to enter the U.S.
While in the U.S. the I-94 status, also known as a non-immigrant status, defines the terms under which non-citizen visitors can temporarily reside in the U.S. All internationals are issued an I-94 status upon entry into the U.S. Border Patrol officer who reviews your visa and other documents at your port of entry, and upon your admittance, writes your immigration status and the length of time you may remain in the U.S. on your Form I-94. Your status in the U.S. is valid up to the expiration date stated on the Form I-94 so long as you do not violate your status according to USCIS regulations.
Maintaining Your Status. It is the responsibility of each non-immigrant to comply with the terms and conditions of their current legal status. If you file a timely non-frivolous application (i.e. application that has a legal basis and is filed before your current status expires) for extension of status or change of status, you will be considered to be in a period of stay authorized by the Department of Homeland Security until the date the USCIS makes a decision on the application. If the change of status or extension of status application is denied unlawful presence only accrues from the original expiration date on the I-94 if the application was frivolous, was denied as untimely filed, or because the applicant engaged in unlawful employment. If it was denied for any other reason unlawful presence only accrues from the time of the denial. Unlawful status accrues in change of status or extension of status applications for persons in duration of status when the applications are denied even if they were filed untimely, were frivolous, or were denied as a result of unauthorized employment.
Unlawful Presence. You can face serious consequences if you are unlawfully present in the U.S. Unlawful presence is defined as presence after the expiration of the period of stay authorized by the Department of Homeland Security, or any presence without being admitted or paroled.
Consequences of Unlawful Presence. In general, any alien (other than an alien lawfully admitted for permanent residence) who was unlawfully present in the United States for a period of more than 180 days but less than one (1) year, voluntarily departed the United States prior to the commencement of proceedings and again seeks admission within three (3) years of the date of such alien’s departure or removal, or has been unlawfully present in the United States for one (1) year or more, and who again seeks admission within ten (10) years of the date of such alien’s departure or removal from the United States, is inadmissible.