Trunked Radio Systems
A trunked radio system is a specialized repeater system with one or more towers, and multiple frequencies, which allows channelized, semi-private conversations between many more groups of users than it actually has allocated RF channels; this is an example of statistical multiplexing. There are many different implementations of trunked business and public-safety radio, using different trunking protocols (like P25 CAI, LTR and Smartzone) and different audio modulations (like FM, NFM, and P25), but they all function in similar ways, and current generation scanners can receive most of them.
Please read the Trunking Basics overview first. Here is UPMan's excellent description on trunking. If you still have questions, feel free to post on the Trunking forum at Radioreference.
Trunked Radio System Types
All forum links go to RadioReference
Sadly some systems are encrypted - and no scanner will ever copy them. A brief listing of these agencies can be found in the Encrypted Agencies article.
The current 800MHz Public Safety band plan change is discussed under Rebanding. You may also find references to what scanners may be supported by following links from the Main Page.
Trunked Radio Terms
Additional Trunking Topics