ADS-B is short for Automatic Dependent Surveillance–Broadcast. It is a satellite based surveillance system. Aircraft position, velocity, together with identification are transmitted through Mode-S Extended Squitter (1090 MHz).
Majority of the aircraft nowadays are broadcasting ADS-B messages constantly. There are many ways you can set up you own receiver and antenna to start tapping into those signals (DVB-T USB stick, Mode-S Beast, Raspberry Pi, RadarScape, etc).