Glossary
SDR (software-defined radio)
A software-defined radio (SDR) hands signal processing to a computer, so a simple USB receiver can listen across a wide range of frequencies.
A software-defined radio (SDR) is a receiver where most of the work — filtering, demodulation, decoding — happens in software instead of dedicated electronic parts.
In practice, a USB stick such as the RTL-SDR picks up the airwaves and sends raw samples to the computer. Free software (SDR++, GQRX) turns them into audio, images or data: FM radio, aircraft beacons (ADS-B), 433 MHz weather sensors or satellites.
An SDR receiver only listens. To get started you need a computer and an antenna suited to the band you want to explore.