Trunking radio is a radio system where a finite number of frequency channels are shared between multiple radio users. This allows support of a much larger number of radio users. A special control frequency is used to determine which frequency a radio should be tuned to.
This all means that following a radio conversation with a software defined radio such as the rtl-sdr can be difficult, as the conversation can hop around multiple frequencies. Fortunately there is software called unitrunker which can listen to the control channel, and determine what voice frequencies need to be tuned to. More information about unitrunker and the signals it can decode is shown on this RadioReference wiki page.
A tutorial on how to set up unitrunker with SDRSharp has been posted here. Essentially, to follow trunked radio conversations you will need two rtl-sdr dongles (or any two software radios), unitrunker and virtual audio cable. One radio will be used for the control channel and unitrunker, and the other will be used for the listening to the voice channel. This can also be done with one software radio, and one hardware radio with discriminator tap if one have one of those. If you are trying to track digital voice communications, it can be done with one dongle and this is discussed in the tutorial too.
Following trunked radios with rtl-sdr is a great development, as only a few years ago an expensive UHF scanner radio would have been required. Also, the scanner radio would have most likely required a hardware modification to expose a “discriminator tap”, which allows audio to be sent to a PC.
Examples of unitrunker
In this video YouTube user bruce buffalo shows unitrunker working with two rtl-sdr devices.
This video by YouTube user opelize shows unitrunker working with one rtl-sdr device, and one Funcube dongle (another cheap sdr).
In this video hamradioscience shows unitrunker at work with one rtl-sdr as the unitrunker control device, and a hardware radio for listening to the audio channels.