The National Interfraternity Conference. It is the international umbrella organization that most, but not all, social fraternities belong to.
IFC is a campus organization, and membership will vary by campus. Chapters of NIC fraternities will almost always be members, but it may also have non-NIC members.
Put it this way -- Sigma Nu Fraternity is a member of the NIC. Your chapter of Sigma Nu is a member of your campus IFC.