Mathematica Notebook
What is a .NB file?
An NB file is a structured document created by Mathematica, an application used for mathematical and scientific computation. It contains text, images, audio, functions, and interactive user interface elements. NB files allow mathematical results to be viewed in an interactive document.
More .NB File Extension Information
Notebook files are saved in a plain text format. Their data is arranged in cells, similar to a standard spreadsheet. Notebooks can be exported to many different file formats, including raster graphic, GIS, compressed, and spreadsheet formats.
NOTE: The "DefaultStyles.nb" file is used as the default style sheet for new notebooks. Notebooks can be opened using the NotebookOpen command within a Mathematica function.
How to open .NB files?
.NB can be using by 3 Flatform (Mac, Windows, Linux).
And can be open by bellow programs list:
Mac  Wolfram Research Mathematica  Wolfram Research CDF Player  Any text editor 

Windows  Wolfram Research Mathematica  Wolfram Research CDF Player  Any text editor 

Linux  Wolfram Research Mathematica  Wolfram Research CDF Player  Any text editor 

Updated at: 20181113