ADS-B is a technology that is meant to improve aviation safety by enhancing surveillance to provide improved situational awareness for aircraft and ground personnel. It enables aircraft to broadcast their position, speed, altitude and heading along with other valuable information. This enables other aircraft, air traffic and ground stations to identify and track other aircraft. This enhances overall situational awareness without requiring everyone to have an active surveillance system (radar) of their own. They only need to be able to receive, decode and process the data.

ADS-B information can be received, decoded and even mapped to provide a real-time traffic picture of the airways.
There are a number of commercially-available ADS-B receiver systems that can be purchased. An inexpensive SDR with an appropriate antenna and a computer with ADS-B software can also be used to receive and decode ADS-B data. ADS-B broadcast data is not encrypted.

The system encompasses many different capabilities and it is being enhanced over time.

There are a number of publicly available websites that track and even map this aircraft data.

ADS-B transmissions occur on 1090.0 MHz for Mode-S Squitter (a periodic burst data broadcast transmission) and 978.0 MHz for UAT (Universal Access Transceiver). The interrogator frequency is 1030.0 MHz.

A simple ADS-B receive station can be built easily with low-cost SDRs and free software.
An example can be seen here  ⓘ An Example of a simple ADS-B station

There are a number of sites that provide free ADS-B information. One excellent site that offers free ADS-B information in the form of maps. These represent one of the most comprehensive collections of online ADS-B data. You can click on the aircraft icons to pop up detailed info on the craft. They aggregate numerous data feeds and shares the information. They also have information that can help you build your own system:

• The ADS-B Exchange aggregates and publishes data from numerous feeds  🔗 ADS-B Exchange

References:

• The FAA's ADS-B FAQ  🔗 ADSB FAQ
• Wikipedia  🔗 Automatic Dependent Surveillance–Broadcast