What is the I-94 form?

Form I-94, also known as the Arrival-Departure Record Card, is a form used by the U.S. Customs and Border Protection (CBP) in order to keep track of the arrival and departure to and from the United States or people who are not U.S. citizens or lawful permanent residents.
It is normally issued by CBP personnel at ports (which includes airports, seaports, and land ports) of entry or deferred inspection sites. It is normally issued electronically at airports and seaports, but can be issued electronically or in paper form at land ports.
For air and sea travel, the carriers collect information about you before you have even departed for the USA and forward this to CBP. Once at the port of entry, an officer of the CBP Office of Field Operations will screen you. If the officer believes, based on evidence and documentation, that you may be admitted to the U.S., then a Form I-94 is issued to you.
At air and seaports, it is issued electronically and your passport is stamped. The form can be retrieved at any time using an online tool. At land ports, only paper forms will be issued if you have not notified the CBP of your arrival in advance, although you may pay a $6 fee and be given an electronic form.
Where officers at ports of entry are unable to decide whether to admit you to the U.S. for any valid reason, a temporary form may be issued which may last up to 30 days. In such scenario, you’ll need to go to a deferred inspection site to complete the process.
A number of persons are exempt from having to use of this form, and this includes people travelling to or through the United States under the Visa Waiver Program (VWP). The ESTA was introduced in 2010 and all VWP now need to obtain ESTA prior to travelling to the U.S.
The system is now paperless, and most travellers entering the U.S. under the VWP who have an approved ESTA will no longer need to fill out the I-94W form, neither will a green I-94W departure card be placed in their passport. The I-94W form, however, will still be required at the land border.