A TEX file is a file type used in document preparation systems, specifically in the widely-used typesetting software, LaTeX. It is a plain text file that contains the markup language used by LaTeX to format and style text.

LaTeX is a document preparation system that was developed for producing high-quality scientific and technical documents such as research papers, books, and articles. It is widely used because it has a simple and easy-to-learn syntax, produces high-quality output, and has many pre-defined features and tools for typesetting various document types.

When working with LaTeX, a user creates a TEX file that contains the document text and the necessary commands to format the text. For example, a user can define the font size, margins, indentation, and paragraph spacing of the document, as well as the formatting of the headings, titles, and captions.

A TEX file can be created using any text editor such as Notepad, TextEdit, or Sublime Text. The file is saved with a .tex extension, which indicates that it is a LaTeX source file.

Once the TEX file is created, it can be compiled into a PDF document using a LaTeX compiler such as pdflatex or xelatex. The compiler reads the TEX file and generates a formatted output that can be viewed, printed, or shared with others.

One of the advantages of using TEX files is that they are version control-friendly. The markup language used in the file is easy to read and understand, which makes it easier for users to collaborate and make changes to the document.

Another advantage of using TEX files is that they are portable. Because TEX files are plain text files, they can be opened and edited on any computer using a text editor. This makes it easy to work on a document from different locations or share it with others.

In conclusion, a TEX file is a plain text file that contains the markup language used in LaTeX, a document preparation system used for producing high-quality scientific and technical documents such as research papers, books, and articles. TEX files can be edited using any text editor, and they can be compiled into a PDF document using a LaTeX compiler. They are version control-friendly and portable, which makes them a popular choice for document preparation in many fields.