Step 1: What's RTL-SDR?
RTL-SDR is the cheapest method to get into the world of SDR (Software Defined Radio) - method of receiving (RTL-SDR have no capability to transmit) and processing radio transmission using your PC. Using it you can receive everything that is transmitted between about 20~2000 MHz depending on second chip on the device.
In brief it can hear i.e.:
- ordinary FM radio stations
- police radio transmissions
- plane and ship tracking informations
- small radio devices like car keys
- probably GPS satellites (as of now there is no confirmed method to do this)
- International Space Station and other space equipment that transmits below 2 GHz
- every other device that you know transmits on these frequencies
Step 2: How to get it?
The most reasonable choice is R820T chip and that tutorial is based on its capabilities. The other good chip is Elonics E4000 but they are actually more expensive ones.