This is one of the new file formats specially suited to sharing scanned documents over the Internet. It is touted as a better way of storing scanned documents - both colour and black & white - when compared to the more popular PDF format.
I came across this file format when I downloaded an e-book which none of the existing programs in my computer could decipher. I have a professional edition of Adobe Acrobat, I thought, with appropriate plug-ins, it could do the job. But I was wrong.
Djvu software is free to download from here. If you are a person who uses scanner a lot, this is a sure shot way of cutting down on your storage expenditure.
More on Djvu can be found in Wiki.