A trunked radio system is communications over a number of frequencies or channels that is controlled by a computer. With a trunked system you may only hear one side of the conversation and not the other or you may only hear one transmission and the system switches to another frequency. You would have to know the algorithm that the system is using to listen to the transmissions. It has almost put an end to the old police scanners as we know them. Also adding digital transmissions to the mix you can really encrypt the transmissions and break them into packets you over the air you hear nothing but noise. Similar to what is going on with your computer as you use the internet. Everything moves from one place to the other via packets.
If you really want a detailed description give this wiki a try.
http://en.wikipedia.org/wiki/Trunked_radio_system