How do I read .djvu files? OR What are DjVu files?

DjVu (pronounced déjà vu) is a file format to store scanned images. Its an alternative to PDF. You can view DjVu files by using a DjVu Reader. Usually books printed on Glossy paper (having a good number of images) are converted to DjVu.
WinDjView and MacDjView is a nice and free DjVu Reader.
You can convert DjVu to PDF using the Universal Document Converter.

For Techies: The images are stored in 'raster' format unlike PDF where the files are stored in 'vector' format. Hence DjVu doesn't use any Fonts.