SQL Server databases have three types of files, as shown in the following table.
DATABASE FILES
Primary - Contains startup information for the database and points to the other files in the database. Every database has one primary data file. The recommended file name extension for primary data files is .mdf.
Secondary - Optional user-defined data files. Data can be spread across multiple disks by putting each file on a different disk drive. The recommended file name extension for secondary data files is .ndf.
Transaction - LogThe log holds information used to recover the database. There must be at least one log file for each database. The recommended file name extension for transaction logs is .ldf.
fonte:https://docs.microsoft.com/en-us/sql/relational-databases/databases/database-files-and-filegroups?redirectedfrom=MSDN&view=sql-server-ver15