The HoloNet, sometimes Holonet, was a Galaxy-wide communications grid that was developed by the Galactic Republic and later used by the Empire.
Messages were routed along millions of non-mass transceivers buried in hyperspace to reach their (sometimes very distant) destinations. This technology enabled data to be sent and received at faster-than-light speeds, ensuring near-instantaneous communication even from far across the Galaxy.
The HoloNet system was an extremely costly expenditure, both for tranceiver placement/maintenance (funded by the government) and for consumer HoloNet equipment. During the time of the Galactic Empire, the HoloNet was strictly controlled, used mostly for Imperial military communications. This greatly inhibited the ability of groups like the Alliance to Restore the Republic to communicate, and arguably also had an isolating affect on the many planets over which the Empire held power.
During the Clone Wars, the Confederacy created a similar network of hyperwave transceivers as a HoloNet substitute.