What is the full form of mdf, ldf and ndf in SQL Server?

SQL Server databases have three types of files: Primary data files The primary data file is the starting point of 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 data files Secondary data files comprise all of the data files other than the primary data file. Some databases may not have any secondary data files, while others have multiple secondary data files. The recommended file name extension for secondary data files is .ndf. Log files Log files hold all of the log information used to recover the database. There must be at least one log file for each database, although there can be more than one. The recommended file name extension for log files is .ldf. SQL Server does not enforce the .mdf, .ndf, and .ldf file name extensions, but these extensions are recommended to help identify the use of the file.