Even if your computer has a meltdown, your stuff is always safe in Dropbox and can be restored in a snap. In fact, if you're using the Dropbox desktop application, your files are backed up several times. The primary copy on your computer's hard drive is synced online and that copy is then backed up again for safety. If you're using Dropbox to sync files between multiple computers, your files are backed up on those computers as well.
By default, Dropbox saves a history of all deleted and earlier versions of files for 30 days for all Dropbox accounts. If you purchase the Packrat add-on for your account, Dropbox will save your files for as long as you have the feature, so you'll never have to worry about losing a folder or file again.
All files synced by Dropbox are encrypted and stored securely on Amazon's Simple Storage Service (S3) over several data centers.