From Uniden:
Conventional scanning is a simple concept. You enter a radio frequency in your scanner's memory, which is used by someone you want to monitor. For example, the police in your area may broadcast on 460.500 MHz, the fire department on 154.445 MHz, the highway department on 37.900 MHz, etc. So when your scanner stops on a frequency, you usually know who it is, and more importantly, you can stop on a channel and listen to an entire conversation. This type of scanning is simple and easy to set up and find.
As the demand for public communications has increased, many public radio users don't have enough frequencies. To use a limited amount of radio frequency efficiently, trunking radio systems were developed.
In a trunked radio system, which contains up to 28 different frequencies, radio users are divided into groups, often called talk groups, and these talk groups are assigned specific IDs. When someone in a talk group uses their radio, a brief burst of data is broadcast before each transmission. The trunking system computer uses this data to temporarily assign each radio in a talk group to an available frequency. If the group using a frequency stops broadcasting or pauses between replies for a few seconds, they are removed from the frequency so another talk group can use it.
Sharing of the available public service frequencies, or trunking, allows cities, counties, or other agencies to accommodate hundreds of users with relatively few frequencies. Following a conversation on a trunked system using a conventional scanner is difficult, if not impossible. Because when there is a short break during the conversation you are monitoring, it is possible that the talk group will be assigned to a completely different frequency in the trunked system. This type of scanning is more complex. Trunk tracking technology solves this problem and allows you to monitor the entire conversation; no matter how many times the system changes frequencies.