Shotwell is a non-destructive photo editor. It does not modify your original photographs. That is to say, if you crop a photo or adjust its colors, the photo file on disc remains untouched. Shotwell stores your edits in a database and applies them on the fly as necessary. This means you can undo any alterations you make to a photograph.
If you want to see what a photo looked like before your modifications, press the Shift key. The original photo will be displayed as long as you hold the key down.
Note that Shotwell can optionally write metadata (such as tags and titles) to photo files. For more information, see the section.