What are NDF Files?

Question

SQL Server uses MDF for data files and LDF for log files, but what are NDF files?

Whats the benefits of these files?

Answer 1

Secondary data files are optional, are user-defined, and store user data. Secondary files can be used to spread data across multiple disks by putting each file on a different disk drive. Additionally, if a database exceeds the maximum size for a single Windows file, you can use secondary data files so the database can continue to grow.

Source: MSDN: Understanding Files and Filegroups

The recommended file name extension for secondary data files is .ndf, but this is not enforced.