Emacs has two helpful features, called auto-backup and auto-save (or at least I call them this way).

Auto-backup is triggered when you save a file - it will keep the old version of the file around, adding a ~ to its name. So if you saved the file foo, you’d get foo~ as well.

auto-save-mode auto-saves a file every few seconds or every few characters (both settings are configurable - auto-save-interval is set to 300 characters by default and auto-save-timeout is set to 30 seconds). The auto-save files have names like #foo# and are deleted automatically when you manually save a file.

Although the modes are definitely useful, many Emacs users find the extra files they create quite annoying(especially since they rarely resort to using them) and disable both feature to get rid of the pesky unwanted files:

;; disable auto-save and auto-backup
(setq auto-save-default nil)
(setq make-backup-files nil)

Even though I’ve never actually had any use of those backups, I still think it’s a bad idea to disable them.1 I find it much more prudent to simply get them out of sight by storing them in the OS’s tmp directory instead.

;; store all backup and autosave files in the tmp dir
(setq backup-directory-alist
      `((".*" . ,temporary-file-directory)))
(setq auto-save-file-name-transforms
      `((".*" ,temporary-file-directory t)))

Now they won’t annoy you constantly, but they will still be around if you need them. Some OSes delete everything in their tmp directories on restart, so if this worries you - consider using another directory.

Prelude keeps auto-backup and auto-save files in tmp by default.

  1. Most backups are eventually useful.