The idea of trunked vs. conventional is to allow for more sets of users to use less frequencies by "sharing" the available resource. Since many "channels" (frequencies) in conventional usage go unused for extended periods, a trunk system has the ability to "fill" that unused space with other users.
Simplest terms -
CONVENTIONAL frequencies - a fixed frequency, typically assigned to a specific set of users for a specific purpose. For example:
154.16 - COUNTY Fire Dispatch
154.25 - COUNTY Fire Response
TRUNKED - A system that typically contains a "set" of frequencies where a computer decides upon which if the set of frequencies a particular conversation will be carried. Typically, the only frequency that is "fixed" in this set is one dedicated to a data stream known as a control channel which all radios on the system monitor to determine which frequency the "talkgroup" they are on is being carried upon.
Instead of fixed frequencies, a "talkgroup" (internal system identifier) is assigned to a channel (i.e. "Fire Dispatch").
Generally, every time a user on specific "talkgroup" want to talk, the computer assigns that "talkgroup" and all users on that talkgroup to (generally) a random frequency in the system. The reply of that "tallkgroup" could be assigned a completely different frequency but will usually still be on the same "talkgroup".
In trunked mode, you can have many times as many "talkgroups" as there are system frequencies. Some systems have dozens of "talkgroups" and only a handful (less than 10) actually frequencies.
