When you get a nonimmigrant visa, you can enter the United States up to the date mentioned in the visa stamp. However, the duration for which you can stay in the U.S. is determined by the date stamped in the Form I-94 at the port of entry.
More details about Visa vs. authorized stay duration.

If you would like to stay in the U.S. for a longer duration than originally planned, you will need to extend your nonimmigrant stay in the U.S. Many people refer to this as extending the visa. However, in reality, you are not extending the date in your visa stamp, but in your I-94 form. You can apply to extend your nonimmigrant stay to continue doing the same types of activities for which you were originally admitted.

You can apply for an extension of nonimmigrant stay in the U.S. if:

• You were lawfully admitted into the U.S. as a nonimmigrant; and
  
  
• You have not committed any act that would make you ineligible to receive an immigration benefit; and
  
  
• There is no other factor that, in the sole discretion of a USCIS officer, would warrant requiring you to depart the U.S. prior to making a reentry pursuant to the same classification.
  e.g., for various reasons, a USCIS officer may determine that you must obtain a new visa before being readmitted to into the U.S.
  
  
• You submit an application for an extension of stay before the expiration date on your Form I-94.
  
  You should file at least 45 days before your authorized stay expires, but USCIS must receive the application latest by the day your authorized stay expires. You may apply at most 6 months before the your I-94 expires.
  
  In order to be able to file for an extension of the stay after the authorized stay has already expired, you must demonstrate that:
  • The delay was due to extraordinary circumstances beyond your control;
    
    
  • The length of the delay was reasonable;
    
    
  • You have not otherwise violated your status, such as working without USCIS work authorization;
    
    
  • You are still a bona fide nonimmigrant; i.e., with some exceptions, you are not trying to become a permanent resident of the U.S.;
    
    
  • You are not in formal removal proceedings for deportation from the country.
    
    
  Other than the limited circumstances described above, if your authorized stay has expired or if you have worked without authorization, you are out of status. You should leave the U.S. as soon as possible to avoid or minimize the possible impact your ability to come back to the U.S. at a later time.

You will not be able to extend your stay in the U.S., if you were admitted in any of the following nonimmigrant categories shown on your Form I-94 (or Form I-94W in case of those who entered on Visa Waiver Program):

• C - Aliens in Transit
  
  
• D - Crewman
  
  
• K1 or K2 - Fiance(e) or Dependent of Fiance(e)
  
  
• S - Witness or Informant (and accompanying family) on terrorism or organized crime, beyond a total of three years
  
  
• Q-2 - Irish peace Process Cultural and Training Program Visitor beyond a total of three years or beyond a total of two years if initially admitted on or after December 10, 2004
  
  
• TWOV - Transit Without Visa
  
  
• VWP - Visa Waiver Program
  
  

You must depart the U.S. on or before the date your Form I-94 (or Form I-94W) expires.

• Employment Based Categories:
  If you are in one of the following employment-based categories, your employer must file Form I-129, Petition for Nonimmigrant Worker, before your Form I-94 expires:
  • E-1 or E-2 - Treaty Traders and Investors
    
    
  • H-1B, H-2A, H-2B or H-3 - Temporary Workers
    
    
  • L-1A or L-1B - Intracompany Transferee
    
    
  • O-1 or O-2 - Aliens with Extraordinary Ability
    
    
  • P-1, P-2 or P-3 - Athletes and Entertainers
    
    
  • Q-1 - International Cultural Exchange Visitors
    
    
  • R-1 Religious Workers
    
    
  • TN-1 or TN-2 - Canadians and Mexican under NAFTA
    
    
  If your spouse and/or unmarried children under age 21 would also like to extend their status, they will need to file a Form I-539, Application to Extend/Change Nonimmigrant Status. All the dependents can be included in one I-539. Both the forms, I-129 and I-539, should be filed at the same time so that both of them can be adjudicated on our about the same time.
  
  
• Other Categories:
  If you are in any of the following nonimmigrant categories, you should file Form I-539 , Application to Extend/Change Nonimmigrant Status:
  • A-3 - Attendants, Servants, Personal Employees of Diplomatic and other Government Officials and Immediate Family
    
    
  • B-1 - Visitors for Business
    
    
  • B-2 - Visitors for Pleasure
    More information
    
    
  • E - Dependents of Treaty Traders and Investors
    
    
  • G-5 - Attendants, Servants, Personal Employees of Foreign Government Officials and Immediate Family
    
    
  • H-4 - Dependents of temporary workers
    
    
  • K-3 and K-4 - Spouse of U.S. Citizen and Minor Child Accompanying/ Following to Join
    
    
  • L-2 Dependents of Intracompany Transferee
    
    
  • M - Vocational Students and Dependents
    
    
  • N - Parents and Children of Certain People who Have Been Granted Special Immigrant Status
    
    
  • NATO-7 - Attendants, Servants, Personal Employees of NATO Representatives, Officials, Employees and Immediate Family Members
    
    
  • O-3 - Dependents of Aliens with Extraordinary Ability
    
    
  • P-4 - Dependents of Athletes and Entertainers
    
    
  • R-2 Dependents of Religious Worker
    
    
  • All "V" categories - Certain Second-Preference Beneficiaries
    
    
  • TD - Dependents of TN visa holders
    
    
  All family members (husband/wife and unmarried children under 21) in the same category (or those with derivative nonimmigrant status) currently can be included on one Form I-539. e.g., a family that all has B2 visa can apply together. Or a primary applicant on F1 visa and the dependents on F2 visa can apply together.
  
  Documents:
  • Application fee. Even if a spouse/children are also included in the same form, the total fee is to be paid just once and there is no extra charge for additional applicants. There is no biometric fee.
    
    Fee Details
    
    
  • Even though the form says you must send original I-94, you must only send the photocopy of the I94 form. Do NOT send the original. You are not required to send the original. If you sent the original by mistake, and if you need it back, you can file form G-884 with USCIS. G-884 is not available online. You have to request it by mail.
    More details.
    
    
  • Copy of all pages of your passport (including blank ones), which is valid for your entire requested period of stay in the U.S.
    
    
  • Look at the Form I-539 instructions for additional list of documents for your specific category.

After the USCIS receives the application, it will look into it to see whether the extension is appropriate. The extension application will either be approved or rejected accordingly.

Some time after the application is received by USCIS, it will mail you a receipt. It will have a receipt number. You can use that receipt number to track your application online.
Online case tracking

If USCIS receives your application before your authorized stay expires, and if you have not violated the terms of your status, and you meet the basic eligibility requirements, then you may continue your previously approved activities in the U.S., including previously authorized work, for a period of up to 240 days, or until USCIS makes a decision on your application or until the reason for your requested extension has been accomplished - whichever comes first. However, during such duration after authorized stay expires, you will not be deemed to be in any nonimmigrant status until the application is adjudicated.

If your extension application is denied, and if at that time your previously authorized stay has already expired, and if you are still in the U.S., you will be considered to have been "out of status" as of the date of your period of stay expired, and will be require to cease employment (if such employment was authorized) and depart from the U.S. immediately. There is no grace period to depart in order not to be considered out of status. The denial letter usually refers to the 30 days grace period. That period is just to depart voluntarily and gracefully before you are forcefully deported out of the U.S. Additionally, any nonimmigrant visa in your passport granted in connection with such classification becomes void at the end of the period of authorized stay. Once your visa is void, you will be required to submit any new visa application at a U.S. consulate in your home country, and not in any third country.