www.Immigrantvisas.state.gov is the official website of National Visa Center, which processes all approved immigrant visa petitions after they are received from Citizenship and Immigration Services in the Department of Homeland Security (CIS) and retains them until the cases are ready for adjudication by a consular officer abroad.
Quick links for you:
Affidavit of Support Processing
Applicant Document Processing
Learn more from http://www.immigrantvisas.state.gov