As of April 30th the I-94 card (the little white card in your passport) is being automated at most airports. Full automation is scheduled to be in place by May 21, 2013. The I-94 used to be issued as a paper card and inserted in the passport as proof of legal status in the U.S. It was then surrendered at the time of departure from the U.S. in order that the departure was recorded. With I-94 automation you must make sure that your passport is stamped with your arrival date, admission status (F-1/J1) and "D/S" for duration of status. Please make sure that you can read the stamp before leaving port of entry. You will then want to print an I-94 for your records via the CBP website:
You will need your I-94 information if you will be employed. Be sure to print it out before applying for your Social Security Number or completing any employment paperwork.
For more information please see the I-94 factsheet:
Follow the below link for video on how to download I-94 cards