OPDS is the Open Publishing Distribution System. It is a protocol, defined at
http://opds-spec.org/blog/2010/08/29/opds-catalogs-v1/, that allows devices and other client programs to discover and use digital content in remote catalogs. Certain e-readers use it to access public online book catalogs. Some of them let you add additional catalogs, which are always referred to with a URL. The protocol first appeared in the Stanza e-reader for the iPhone (
http://www.lexcycle.com/).