A visa stamp is issued by a U.S. embassy or consulate outside the United States. The visa indicates that the visitor is eligible for entry and reentry to the United States in a specific immigration category (F-1, J-1, H-1B, O-1, etc..) The validity length of the visa varies greatly based on agreements between the US and other countries. Most visas allow for multiple entries.
The I-94 number is the “Arrival/Departure Record” issued by U.S. Customs and Border Protection (CBP) whenever you enter the United States. This record confirms that you have been legally admitted to the U.S. in a specific visa status, and for a duration of time. For F and J visa holders the amount of time is indicated by the notation “D/S” which means you can be in the U.S. for the Duration of Status of your student or scholar program. If you arrive by air the CBP will confirm your immigration status by swiping your passport. If you arrive by land you may be issued a paper I-94 record.
Note: If you have an F or J visa and your I-94 record does not indicate “D/S” you should speak to an advisor immediately.