I-94 Card (small white card usually stapled in your passport) is known as an Arrival/Departure Record. All students and their dependents are issued this card upon entry to the U.S. in all visa classifications. This card is used to record the period of authorized stay in the U.S.
You will give the I-94 card to the airline staff when you depart the U.S. (except when you travel to Canada or Mexico for 30 days or less). You will get a new I-94 when you reenter but the pre-printed number will be crossed out and the number from your original I-94 card will be written in the space below it.