In many countries, a visa is directly tied to the person's immigration status. However, in the United States, a visa stamp expiration date and the authorized duration of stay in the United States are two entirely different things.

A visa is permission to apply to enter the United States. Citizens of most foreign countries generally require visas to enter the United States. Visa is a document affixed to a page in the passport.

Sample visa stamp

Most visas are issued at one of the Department of States embassies and consulates abroad and the person must apply there first for a visa, if required.

For this discussion purposes, a visa is a nonimmigrant visa issued to person that wants to come to the US temporarily for a specific purpose such as tourists, business people, students, temporary workers etc. On the other hand, an immigrant visa allows the persons to stay and work in the US permanently.

A visa is not a permission to enter the United States. A visa does not guarantee entry into the US. A visa simply indicates that a US consular officer has reviewed your application at a US embassy/consulate, and he/she has determined that you are eligible to travel to the port of entry (POE which can be an international airport, a seaport or a land border crossing) for a specific purpose such as tourism, study, temporary work, business meeting etc.

A visa stamp has an expiration date. Unless voided or canceled, a visa is valid until the expiration date. A visa validity is the period from the visa issuance date to visa expiration date.

As long as the visa is valid, a person can apply at the port of entry for entering into the US, even on the last day of the visa expiration date.

A single entry visa allows the person to apply to enter the US once.
Multiple entry visa allows the person to apply to enter the US multiple times while the visa is valid, for travel for the same purpose. You don't have to obtain new visa every time you travel to the US as long as the visa is valid for the entry.

You can't use the same visa to enter the US for a purpose different than for which it was originally issued. If you have a tourist visa (B2 visa), you can't use it later to enter the US at a later date. You need to first obtain a student visa (generally F1 visa) before you can apply to enter the US to study.

A visa validity can sometimes be voided or canceled depending upon certain circumstances. If you overstay (stay beyond your authorized stay), as provided by the date in the I-94 at the port of entry or through extension, your visa will be voided or cancelled automatically. Read clause 222(g) for more details.

However, if you have filed a timely application for extension of stay or a change of status, and that application is pending and not frivolous, and if you did not engage in unauthorized employment, this normally does not automatically cancel your visa. However, if your application for extension or change of status is rejected and if you are still in the United States at that time, you immediately incur an unauthorized stay and your visa will be voided/canceled automatically.

If you have applied for an adjustment of status to become a permanent resident alien ("green card" holder), and if you are not on a valid H or L nonimmigrant status, you must apply for an Advance Parole before leaving the US. Also read EAD/AP vs. H/L.

Visa expiration date has nothing to do with the authorized length of your stay in the US for any given visit.

A US immigration officer of the Department of Homeland Security (DHS) determines whether to allow you to enter the United States and for how long you can stay for any particular visit.

Before entering the US, usually in the flight, you will be a given a small white form, called Form I-94, Arrival-Departure Record. Look at admission into United States for more details.

When you arrive into the US, an immigration officer at the port of entry, records either a date or "D/S" (duration of status) on your I-94 card. He/she also records the "class of admission" (which corresponds to the visa class) such as B2, H1 etc. An immigration officer is an officer of the Department of Homeland Security's Border and Transportation Security.

An officer will keep the top (arrival) portion of the I-94 form with him/her and staple the bottom (departure) portion in your passport.

If your I-94 contains a specific date, you are authorized to stay in the US until that date. You may even stay in the US on the last day. If you are entering the US as a tourist or H/L worker, your I-94 will contain a specific date.

Some student, exchange program participants, and certain temporary workers (e.g., foreign diplomats) will be admitted for "duration of status". If you have "D/S" in your Form I-94, you can legally stay in the US as long you continue your course of studies, remain in your exchange program or qualifying employment. D/S notation allows the educational institution to extend or transfer one's legal immigrant status as a student without any additional applications to USCIS.

If you are traveling to the US on the Visa Waiver Program, you will receive Form I-94W, Nonimmigrant Visa Waiver Arrival-Departure Record and it is a green color card. (It is NOT "green card". "green card" is specifically referred to "permanent resident card" which is an entirely different type of card.)

The date on the Form I-94, Form I-94W or D/S notation is the official record of your authorized length of stay in the US and it is entirely different from the visa expiration date.
e.g., Visa may be expiring after 7-8 years or it might have already expired 3-4 months back, but you are allowed to stay for next 2 months in the US.

If you would like to stay in the US beyond the date recorded in the I-94, you must apply for an extension of stay, by filing Form I-539 with the USCIS. USCIS has the authority to grant or reject the extension of stay. If extension of stay is granted, you will get an approval notice along with the new I-94 form with the advanced expiration date.

If a person's legal nonimmigrant status is valid because of extension of stay, change of employer (H1 transfer) etc., but if the visa stamp is expired, a person can legally stay in the US until the authorized stay. However, if such person goes outside the US, he/she must get a visa stamp for the new nonimmigrant status before being allowed to enter the US again. Currently there is no way to apply for a visa inside the US.

Application for change of nonimmigrant status, e.g., from a student status to H1 status, must be filed with the USCIS by filing Form I-539. While the person may change to a valid nonimmigrant status such as H1, without having a new visa stamp in the passport, if such person goes outside the US, he/she must get a visa stamp for the new nonimmigrant status before being allowed to enter the US again. Currently there is no way to apply for a visa inside the US.

• H1 workers should be aware of the period of stay granted on the I-94 as immigration officers tend to grant the period authorized stay only up to the validity of the visa when entering under the portability rules.
  
  
• If the person is entering the US on a visitors visa, it is possible to enter the US even on the last day of the visa stamp, and get 6 months stay authorized at the port of entry.
  
  
• If the person has 10 years multiple entry tourist visa, that does not mean the person can stay in the US for 10 years. Such person will usually be given a 6 months stay authorization at the port of entry which can be extended up to another 6 months by filing an extension. That means, such persons must go back to the home country after staying maximum of 1 year in such cases. But that does not mean that you stay in your home country for 1 month and again come back to the US and try to stay in the US for 6 + 6 months. And again, try to do the same thing. While there is no official rule regarding how long a person should stay outside the US and how long inside the US, if the person tries to spend more time in the US compared to outside the US (some people try to spend as much time as possible in the US), that is not consistent with the purpose of the tourist visa. Tourist visa is given to the person to tour the country and not stay here all the time. If you are caught doing this, your next entry may be limited to 1 month or 1 week. Or you may be denied entry altogether and sent back to your home country. People have been sent back from the airport and it regularly happens to such abusers.