Also known as Form I-94, Arrival-Departure Record. The Department of Homeland Security, Customs and Border Protection official at the port-of-entry gives foreign visitors (all non-U.S. citizens) an Arrival-Departure Record, (a small white card) when they enter the United States. Recorded on this card is the immigrant classification and the authorized period of stay in the U.S. This is either recorded as a date or the entry of D/S, meaning duration of status. It is important to keep this card safe because it shows the length of time you are permitted and authorized by the Department of Homeland Security to stay in the U.S. It is best kept stapled with your passport, kept in a safe place. The visitors return the I-94 card when they leave the country. The I-94W, Nonimmigrant Visa Waiver Arrival-Departure Record (green card) is for travelers on the Visa Waiver Program.
Source: U.S. Department of State.