On most operating systems, rewriting a file automatically destroys all
record of what the file used to contain. Thus, saving a file from Emacs
throws away the old contents of the file--or it would, except that
Emacs carefully copies the old contents to another file, called the
backup file, before actually saving. (This assumes that the
variable make-backup-files is non-nil. Backup files are
not written if this variable is nil.) Emacs does not normally
make backup files for files in `/tmp'.
At your option, Emacs can keep either a single backup file or a series of numbered backup files for each file that you edit.
Emacs makes a backup for a file only the first time the file is saved from one buffer. No matter how many times you save a file, its backup file continues to contain the contents from before the file was visited. Normally this means that the backup file contains the contents from before the current editing session; however, if you kill the buffer and then visit the file again, a new backup file will be made by the next save.
You can also explicitly request making another backup file from a buffer even though it has already been saved at least once. If you save the buffer with C-u C-x C-s, the version thus saved will be made into a backup file if you save the buffer again. C-u C-u C-x C-s saves the buffer, but first makes the previous file contents into a new backup file. C-u C-u C-u C-x C-s does both things: it makes a backup from the previous contents, and arranges to make another from the newly saved contents, if you save again.
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