A fiancé(e) visa (or K-1 visa) is technically a nonimmigrant visa. However, the Immigrant Visa Unit of the US Embassy in Buenos Aires handles the processing of this visa, because the application process is very similar to that of any other immigrant visa. The fiancé(e) visa is for foreigners who wish to marry a U.S. citizen in the United States and then become legal permanent residents without having to leave the United States. K-2 visas are for the children of K-1 applicants.
For more information about K visas, please click here.