Your passport, issued by your country of citizenship, should always be valid at least six months into the future at the time of your arrival into the U.S.
The visa is a stamp placed in the passport to allow an individual to enter a particular country, such as the U.S. The visa indicates the purpose of someone's visit, and it is valid for a specified number of entries to the United States: one, two, or "multiple," until the expiration date. The validity period of a visa stamp pertains only to the period during which it may be used for admission to the U.S. - it does not indicate how long a person may stay in the U.S.
The form I-94 is the Arrival/Departure Record issued by the U.S. Customs and Border Patrol (CBP) when you enter the United States. Beginning April 30, 2013 CBP will automate this process by issuing digital I-94 records at all airports and seaports. Instead of receiving a paper I-94 record when you enter by air or sea, your I-94 information will now be made available online. It is important to note that USCIS and other government departments in the U.S. will continue to require students to submit a paper copy of Form I-94 when requesting certain benefits. Therefore, the OGS advises you to print and save a paper copy of the electronic I-94 soon after arrival.
Note: you will have this form only if a petition was filed with USCIS. The form I-797 is a multi-purpose form issued by the U.S. Citizenship and Immigration Services (USCIS). Most commonly it is used as a receipt notice or an approval notice related to an application submitted to USCIS. An I-797 that serves as an approval notice may or may not include an original I-94 card on the bottom quarter of the page. I-94 cards are generally included when the application requested an extension or change of status. Individuals should retain the original I-797 form for travel purposes and for their own records. Do NOT lose the original I-797. To replace the form requires an application with fee to USCIS.