Dropbox is a cloud storage service which allows you to backup photos, videos and documents to share them with others. When you download and install Dropbox on your computer it creates a special folder (Dropbox folder) on each computer for storing the photos, vidoes and files. Changes made to one Dropbox folder (on your computer) will reflect in other Dropbox folders (on other computers) as well. This is done via Dropbox automatic sync. If you are using Dropbox Desktop Application, your files are backed up several times. The first copy on your computer's hard drive is synced to your Dropbox account online and backed up.
Dropbox saves a history of all deleted files for 30 days for all Dropbox accounts. It offers you to purchase a 'Extended version History' add-on feature which is used to recover a deleted file at any time within a year of an edit or deletion made.
When you backup photos, videos and files online to Dropbox, those are in encrypted format. It keeps your files in secure storage servers across several data centers.