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 make up all the data files, other than the primary data file. Some databases may not have any secondary data files, while others have several secondary data files. The recommended file name extension for secondary data files is .ndf.

Transaction Log file

This file holds all the log information that is 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.