ADS-B is a global standard that most all aircraft use to identify each other to other aircraft as well as to ground controllers to maintain safety and efficiency in the air and on the ground.
ADS-B transponders broadcast at 1090 MHz and because they are not encrypted, they are easily monitored by hobbyists using inexpensive hardware and open source software.
The ADS-B system augments other aircraft communication systems such as the common 118 to 136 MHz and 225-400 MHz frequencies commonly in use for voice or data communication by pilots, airports and regional air traffic controllers.
A simple $20 Software Defined Radio (SDR) connected to a computer or Raspberry Pi with the appropriate antenna and software is all you need to get started.
They have to scrub out certain details from the public though but you can gain much more detail by creating your own local receiver for your own enjoyment to see much more.
You can learn more about what is needed and how to create your own ADS-B monitoring station easily via a few quick internet searches, Members of the HVDN can even assist you in answering some questions.
The HVDN system is built upon common and easily sourced equipment and offers lots of advanced searching and filtering tools to our supporting members who gain a username/password to access the system.