Nolo's Plain-English Law Dictionary
Until 2013, this was a small green or white card given to all nonimmigrants when they enter the United States. The I-94 is still used, but getting a copy is now an online process for most nonimmigrants (with exceptions; those who still receive paper I-94s include refugees, asylees, parolees, people who arrive at a land port or using transport other than a commercial air or sea carrier, or those in whose cases CBP decides it’s necessary to give an I-94 for some other reason).
To download one's I-94, the nonimmigrant needs to go to the U.S. Customs and Border Protection (CBP) website). The I-94 serves as evidence that a nonimmigrant has entered legally. It contains a date indicating how long the nonimmigrant may stay for that particular trip. (It is this date--and not the expiration date of the visa -- that must be followed in determining when the immigrant must leave the United States.) A new I-94 with a new date is issued each time the nonimmigrant legally enters the United States. Canadian visitors are not normally issued I-94s.