You must keep your passport valid at all times. The International Student Services Office can supply you with the address and phone number for your country's consulates and embassy. The visa stamp put in your passport by the U.S. consul abroad is needed to enter the United States. The visa shows the latest date on which you can apply to enter this country. It does not show how long you can stay here.
American visas come in two formats.
The traditional one is a red and blue stamp. Much of the information on the computer-printed visa is in code. If it is necessary for you to renew your visa in order to re-enter the U.S., you may do so by visiting the U.S. consul in the country to which you are traveling. The I-94 is usually stapled onto the U.S. visa page of your passport. You must leave the U.S. by that date, or you must apply to extend your stay.