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 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.
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.
Note: Canadians do not need visa stamps to enter 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.