All U.S. employers are required to complete and keep a file copy of the Form I-9 for every person hired within the United States, whether a citizen or non-citizen hire. This form allows the employer to examine the eligibility for employment within the U.S., and verification documents as presented by each person hired.
The U.S. Citizenship and Immigration Services, has a great blow-by-blow of the I-9 requirement, including access to the form itself.
The I-9 Form was recently revised on 2/2/09 in order to revise the list of acceptable documents submitted as part of the I-9 process. From this point forward any documents which have expired will not be acceptable to the process. Previous forms of the I-9 can no longer be used.
Remember that the I-9 process takes place AFTER you have made your selection in the hiring process and an offer has been accepted. On the employee's first day of work, all HR paperwork is completed, with the I-9 being a part of that paperwork. You must retain the I-9 form in your files for three years.