It gives you free 2GB of online storage always in sync with a DropBox folder on all your PCs or Macs. You can even share folders online with other people.
If you subscribe through this link, we both would get 250MB extra for free!
DropBox is not an Online Backup solution. For that, I use Mozy, but lately it feels slow and backup uploadtime is really important; so I'm giving a chance to iDrive. Its faster and has better UI. And it allows me to sync my two Macs to a single account.
So I ask you: Is your PC or Mac backup securely stored on a place outside your home? A local USB Disk is not enough.