Amateur radio operation on the Internet-enabled D-STAR digital voice and data protocol.
D-STAR is a packet radio-based system that uses digital transmission modes to encode and send voice and data. D-STAR repeaters can be linked over the Internet to provide coverage across wide geographical areas.
In addition to repeaters, D-STAR also uses Internet-only connection points known as a "reflector". Reflectors allow several repeaters to be linked so that when one user transmits into the repeater, their voice is broadcast to all linked repeaters as well as anyone connected to and listening on the reflector.
Almost all D-STAR compatible equipment is sold by Icom. For more information, Icom offers a general introduction and some background material on their website: http://www.icomamerica.com/en/products/amateur/dstar/dstar/
It is also possible to connect to the D-STAR system over the Internet without a radio with the use of a device called a "DV Dongle". This device is sold at http://www.dvdongle.com and contains the proprietary voice codec chip used in D-STAR radios. The dongle can be used with the DVTool software available from http://opendstar.org/tools/ in order to connect to D-STAR repeaters and reflectors without using a radio.