When you decide to travel to the United States, whether for business or pleasure, you’ll need both a passport and a visitor visa.

A visa issued by the U.S. consulate or embassy allows you to travel to the United States, but the immigration officer at the port of entry is the person who allows you to enter the country and decides how long you may stay.

Your visa expiration date

Your visa will state how long you are able to travel to the country. This length of time varies by case and depends on the decisions of the consular or embassy officials who process your application.

Note that the expiration date of your visa is not the same as the time you are allowed to remain in the country during each trip. For example, a visa that’s valid for 10 years allows you to travel back and forth to the United States for that period, but doesn’t mean you can stay in the country for the entire time.

Your length of stay in the United States

Upon entering the country you need to complete and submit an immigration I-94 form (which registers arrivals and departures). The forms are available at any air, sea or land port of entry.

The immigration officer will ask you a series of questions about the purpose of your visit. The agent will also stamp your passport with the date of arrival and write on your I-94 the length of time you can stay in the country — for example, 60 days. This length of time indicates your legal presence in the United States and may be extended if necessary.

How to extend your stay

When asking for an extension you need to verify that you haven’t exceeded your authorized stay.  For instance, if you arrived as a tourist but want to become a student, you must submit an application to change your status before your stay expires.

To make your request you must complete an I-539 form. Be sure to do this well in advance.

If you stay in the country beyond the authorized time, your presence will be considered illegal, and you could lose the privilege of returning to the United States or your visa renewal may be denied.

What to do if your documents get lost or stolen

If you lose your travel documents, or if they get stolen, follow these suggestions:

Source: http://blog.usa.gov/post/78109813729/know-the-difference-between-your-visa-expiration-date