GNU Emacs Manual. Node: Files
Chapter 12: File Handling
The operating system stores data permanently in named files. So
most of the text you edit with Emacs comes from a file and is ultimately
stored in a file.
To edit a file, you must tell Emacs to read the file and prepare a
buffer containing a copy of the file's text. This is called
visiting the file. Editing commands apply directly to text in the
buffer; that is, to the copy inside Emacs. Your changes appear in the
file itself only when you save the buffer back into the file.
In addition to visiting and saving files, Emacs can delete, copy,
rename, and append to files, keep multiple versions of them, and operate
on file directories.