Backup
When you turn on the backup function in the General preferences, a backup version of your diagram is kept in the file as you work on it. In the case of a crash, power failure, or other nasty event, the backup version remains in the file. When you open the file again, you'll be asked whether you'd like to use the last saved version or the automatically backed-up version.
There are a couple of catches: If you have a diagram open with unsaved changes, and you send the file to someone else, the file will contain your unsaved changes in the backup version; the other person will be asked whether to use the backup version. Please save your files before sending them around. A similar effect could happen when two people are accessing one file at the same time, over a network. In that case it's best to turn off the feature so as to avoid colliding with other people's changes.