The idea is simple: upload your photos to their service and they’ll store them for you securely for as long as you like. Then if anything happens to your originals, you can use the service to retrieve the backups.
There are two service levels to choose from. A limited free service (a trial really) or a $40/year subscription. With either account you get 25 GB of storage. The free trial service only backs up low-resolution JPG images. The paid subscription service will backup full-size images of every type including RAW files (JPG, TIF, PNG, RAW, CRW, DNG, NEF, …). The paid subscription also backs up office files (DOC, XLS, …), movie files (MOV, AVI, …), financial files from Quicken and Microsoft, and music (AAC, MP3, …). Pretty cool.
