Wht is I-94?

The I-94 is a U.S. visitor's arrival and departure record. This card will be giving to you upon entry to the U.S. Please keep this card as you will be asked to turn it into DHS when you depart. This is a very important document and always keep it stapled with your passport near your visa is stamped.
Your visa expiry date and I-94 card expiry date may be same or different. But incase your I-94 arrival card has different expiry date then this is the date until which you can live in USA. Stay beyond this date is absolutely illegal and is a serious crime.
Many a time it happens that people have H1 visa expiry date of 1 or 2 years down the line and the I-94 card expiry is 3 years down the line. Means, if you do not leave the country then you can live in USA until the expiry of the I-94 card even though your H1 visa has expired earlier.
But once you leave the country anytime, then you will have to hand-over the I-94 card to the INS officer at the airport and you need to have a valid H1 Visa before again entering the USA. For the second time, a new I-94 card will be issued to you and this will be your new date until which you can live in USA.
Incase you loose this document during your tenure in USA, immediately get back to your company HR and attorney to get a new copy of the document. This should be done as soon as possible.