ADS broadcast (ADS-B) is transmitted from the aircraft transponder continuously whilst it is powered on. It is different from ADS-C which is transmitted to a specific ground station using SATCOM or VHF etc at specific times determined by the 'contract'.
ADS-B sends out various aircraft parameters including position every few milliseconds which is decoded by a ground station. It is distinctly different from a radar that picks up position manually and SSR information.
The ADS broadcast is not encrypted and can be picked up by anyone with a 1090mhz receiver which includes ground stations, spotters, and even other aircraft that have ADS-In capability. This can be displayed on the ND etc.
ADS-B therefore has the advantage that no ground based radar receiver is required and the only device required to present an accurate radar picture is a single tuner capable of receiving on 1090mhz.
Edited to add note that the ADS-B will only be picked up within range of the transponder which is typically a couple of hundred miles....ADS-C using satcom, such as is used Oceanic will work when in range of satellites.