The I797 is a form that is used by the United States Citizenship and Immigration Services. The form is used to show approval of immigrant visa petitions.
US Immigration http://uscis.gov/graphics/index.htm
An i979 approval is a notice of action form. This can be used to let someone know that a petition has been granted or if an alien worker petition has been granted.
USCIS uses numerous types of Form I-797 to communicate with customers or convey an immigration benefit. Form I-797 is NOT a form you can fill out. The chart below gives a brief description of each:Form NumberDescriptionI-797, Notice of ActionIssued when an application or petition is approved.I-797A, Notice of ActionIssued to an applicant as a replacement Form I-94.I-797B, Notice of ActionIssued for approval of an alien worker petition.I-797C, Notice of ActionIssued to communicate receipt of payments, rejection of applications, transfer of files, fingerprint biometric, interview and re-scheduled appointments, and re-open cases.I-797DAccompanies benefit cards.I-797E, Notice of ActionIssued to request evidence.I-797F, Transportation LetterIssued overseas to allow applicants to travel.