Automatic dependent surveillance-broadcast (ADS-B) is a cooperative surveillance technology for tracking aircraft. --Wikipedia
This is one of the ways in which the airplanes report their basic flight parameters (identification, current position, altitude, speed, etc) to the Air Traffic Control. All of this happens over radio broadcasts (completely public) at 1090MHz - at lest this is the frequency we are interested in. The full Wikipedia article is quite interesting and contains more information.
If you have ever seen the flightradar24 website before, you have possibly thought (I know I did) about how cool would it be to do it yourself - own one of those virtual radars and maybe even feed the data into flightradar24 to improve their coverage (and get premium account for free)? If you like this idea, you will like the results your own radar station will produce - in general it would be something like this
All of that is easy to build and the cost is really minimal, using either a PC or a Raspberry Pi as I describe in this post.