What is an I-94 Card?

The I-94 card is filled out by every nonimmigrant who comes into and leaves the United States. It basically proves when and where you entered, and when and where you left. In addition to the card, your passport is stamped by the immigration official at the airport or port.
This card/stamp is important because if you ever want to adjust your status you may need to prove you entered legally and without a stamp/card that could prove very difficult.

You can find a form at www.uscis.gov/propub/docView/afmid/dat/I_94.pdf