If you or a loved one is outside the United States and thinking about immigrating to the United States, then you should be aware of the National Visa Center (NVC).
The NVC is a visa processing center operated by the US Department of State. After the U.S. Citizenship and Immigration Services (USCIS) approves an immigrant visa petition, the USCIS forwards the petition to the NVC. The NVC is responsible for collecting visa application fees and documentation and retains the visa petition until the case is ready for adjudication by a consular officer abroad. Petitions may remain at NVC for several months or for many years depending on the visa category and country of birth of the visa applicant.
In summary, the NVC is an intermediary between the approval of an immigrant petition by the USCIS and final adjudication by the local consulate after an interview is held.