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 actually encompasses many different capabilities and continues to be enhanced over time.

Internationally and, in the U.S., aircraft that fly above 18,000 feet must use the 1090MHz ADS-B ES service. Almost all commercial aircraft will use 1090MHz. In the U.S, aircraft that operate below 18,000 feet can use the ADS-B UAT system which broadcasts at 978Hz. This system is used by the very large population of General Aviation aircraft in the U.S.

ADS-B transmissions occur on 1090.0 MHz for Mode-S (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. These are the most comprehensive collections of online ADS-B data. They provide interactive maps that display the aggregated data. You can click on an aircraft icon on their maps to pop up detailed info on the craft. They depend on numerous participants providing their own data feeds which they then aggregate and share. Providers like ADS-B Exchange and FlightAware also have information that can help you build your own system:

• The ADS-B Exchange aggregates and publishes flight data  🔗 ADS-B Exchange
• FlightAware also aggregates and publishes flight data  🔗 FlightAware

References:

• RTL-SDR Blog's ADS-B Tutorial (old but still good)  🔗 RTL-SDR Tutorial: Cheap ADS-B Aircraft RADAR
• The FAA's ADS-B FAQ  🔗 ADSB FAQ
• Wikipedia  🔗 Automatic Dependent Surveillance–Broadcast