Form I-94 is nothing but a white or green form, issued to non-immigrants, by the Customs and Border Protection(CBP) officers and this is also known as the arrival/departure record. Foreign nationals who get into the United States on non-immigrant visas will have to fill out this form, while they arrive at a US port of entry and this applies to travelers who get into the country by air, sea or land. During immigration inspection, this form must be presented to a CBP officer and the CBP officer will return the lower portion of this form, the departure record, to the non-immigrant, if he is admitted.
Such non-immigrants must return those departure records to CBP officers while they leave the country. Their departure will not be recorded, if they fail to return that document. Similarly, they may not be able to prove that they had departed at the end of the authorized period of stay. They are likely to get into trouble when they try to apply for US visas again and the CBP officers might conclude that they had remained in the country illegally, beyond their authorized stay. In such cases, their visas will be canceled. If they lose their departure records before their departure, they can file Form I-102, Application for Replacement/Initial Non-immigrant Arrival-Departure Document and obtain new documents.
If the people who travel to the United States under the Visa Waiver Program(VWP), fail to return their departure records and fail to record their departure, they may not be able to get into the country again under this program and they may be required to obtain visas to travel to America. If you are a VWP visitor, see to that you record your departure, while you leave the country. If not, you may not be granted entry into the United States under the Visa Waiver Program, again. However, you may mail your Form I-94, to the DHS, if you failed to return it while leaving the country.
People who depart by airlines or cruise ships may use their boarding passes to establish that they have not overstayed their visas and those passes will help them to travel to the United States, next time. However, they may mail their departure records along with some other supporting documents such as their boarding passes, copies of passports with departure stamps or some other documents that will help them to prove that they had departed the United States and that they are in another country.