An NDF file is a secondary data file used by Microsoft SQL Server. SQL Server databases are typically stored across multiple files for performance and management reasons. The primary data file, with the .MDF extension, contains the database metadata and pointers to the other files. NDF files, on the other hand, store the actual data tables and indexes. A single SQL Server database can have multiple NDF files, allowing the database to grow beyond the size limitations of a single file system volume. This is particularly important for large databases that require significant storage capacity. NDF files are an integral part of the SQL Server database structure and are essential for the proper functioning of the database. They are managed by the SQL Server Database Engine and are not typically accessed directly by users or applications. The use of NDF files allows for better organization, scalability, and performance of SQL Server databases.