A WebSDR is a Software-Defined Radio receiver connected to the internet, allowing many listeners to listen and tune it simultaneously. SDR technology makes it possible that all listeners tune independently, and thus listen to different signals; this is in contrast to the many classical receivers that are already available via the internet.
WebSDR was first conceived as a means to make the 25 m radio telescope at Dwingeloo available to many radio amateurs for EME reception. In order to test a preliminary version of the software without using the 25m dish, a shortwave WebSDR was set up on Christmas Eve 2007 at the radio club of the University of Twente. After further development, its existence was publicly announced in April 2008. Interest for the project has been large since then, and many amateurs worldwide have expressed an interest in setting up their own WebSDR server. In November 2008, a beta testing phase has started with a few selected stations. By now, the software is made available to anyone serious about setting up a server; see the FAQ for information on this.
A WebSDR server consists of a PC running Linux and the WebSDR server software, a fast internet connection (about a hundred kbit/s uplink bandwidth per listener), and some radio hardware to feed antenna signals into the PC. This radio hardware is typically a quadrature mixer connected to the PC's soundcard, like the popular SoftRock kits.