Automatic Dependent Surveillance–Broadcast (ADS–B) is an advanced surveillance technology that combines an aircraft’s positioning source, aircraft avionics, and a ground infrastructure to create an accurate surveillance interface between aircraft and ATC. ADS–B is a performance–based surveillance technology that is more precise than radar and consists of two different services: ADS–B Out and ADS–B In.

ADS-B Out works by broadcasting information about an aircraft's GPS location, altitude, ground speed and other data to ground stations and other aircraft, once per second. ADS-B Out airspace and equipment requirements are contained in 14 CFR § 91.225 and the equipment performance requirements are contained in §91.227. ADS-B In provides operators of properly equipped aircraft with weather and traffic position information delivered directly to the cockpit.

Here you can find links to ADS-B regulations and guidance, as well as detailed information on the airspace and equipment requirements and capabilities.