ORB is a format used by the Original Book Reader. This is a proprietary format but the eBook reader that can be freely downloaded.
The reader is available at http://originalebook.com. It is a fixed format eBook reader that claims to support reading like a regular book. The reader is currently only available for Windows but a Mac OS X version is planned.
 The format
The internal format to this file is SWF files inside a ZIP container. You can rename the .orb extension to .zip and unzip the contents. The files inside may have a .gau extension however the extension can be changed to .swf and then read using any SWF tool.
 For more information
Discuss this format in our forum