Text file:
sequence of ASCII/Unicode characters: simplest format.
 No hidden formatting like in Word. Except for end-of-line character(s).
Each character is one byte. 
Viewable in any text editor, e.g. Notepad or Mac TextEditor,  and web browser. 	
"universal and eternal"

Common extensions of text files:
.txt  .dat
Web pages: .html  .css   (and dynamically generated HTML)
Source code of software: .py .c .cpp .js .bas .bat ...
Data interchange formats: XML JSON 

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Notepad:

Format | Font...   one font and size. How the characters are displayed.
                   A setting of Notepad: all files will be shown in this font. 
                   Typically a fixed-width font is best for text.
View | Status bar    On by default. To see line and column numbers in the status bar.

View the contents of the file.  Ctrl+Home, Ctrl+End, PgUp, PgDn
Search:  Edit | Find...  Ctrl+F
Goto line:   Edit | Go To...
Edit it.    Select: Ctrl+A, mouse drag, Shift+arrow, double-click word, W11: triple click line 

Notepad cannot sort or filter.


A file that is not a text file is categorized as a binary file.

Notepad can open any type of file and will "view"/diplay it as a sequence of ASCII characters.
Try: right-click a .docx, .xlsx, .png file   Open with...  choose Notepad
    OR  from within Notepad, Open any file.