At the port of entry into the U.S., the immigration officer places a small white card, Form I-94, Arrival-Departure Record on your passport. Often the Form I-94 is handed out on the plane shortly before landing. The Form I-94 indicates the non-immigrant's classification and any endorsements made by the immigration officer.
The Form I-94 indicates the place and date of entry of the non-immigrant's admission to the United States and the initial period of authorized stay. Noted on the Form I-94 for students and their dependents is "D/S", for duration of status. If you have "duration of status" or “D/S” on your Form I-94, you may remain in the U.S. as long as you appropriately continue your course of study or remain in your exchange program or qualifying employment.
For further information, please visit the following webiste: U.S. Department of State