On a simple conventional network (such as the CB network), end-users are arranged by the frequency they have selected on their radio.
Anyone can talk to anyone else who has the same frequency selected.  Everyone who is tuned into the same frequency forms a large ‘talkgroup’.  Everytime you want an extra ‘talkgroup’ you need to install another base station with a different frequency.

		Figure 1: Conventional example

In Figure 1, three different groups of people are sharing Channel 2.  If Group H is talking, then Groups F and G must wait before they can talk, even if none of the other groups (A to E and I to L) aren’t talking.

		Figure 2: Trunked example

On a trunked network, end users are arranged in ‘talkgroups’.  A talkgroup is a bit like a conference facility.  Only people with the correct authorisation can participate in the discussions taking place.  The radio holds a list of authorised talkgroups for that user.  The user can select the talkgroup they want to use or listen to as long as it’s in their radio.
In Figure 2, all talkgroups sit on the control channel when not in use (i.e. no conversation taking place).  When someone wants to talk, the talkgroup is dynamically allocated an available channel for the duration of the call (takes around 0.5 second to set up).  At the end of the call, the talkgroup moves back to the control channel until someone wants to talk again, at which point they are allocated the next free channel.  In the above example, Group H is currently talking.  If Group F wanted to make a call, they would be allocated Channel 3.  In reality, trunked systems generally have more voice channels serving a lot more users. 
The more channels there are, the more efficient the network becomes.  Figure 3 shows a graph of the number of users per channel versus queueing delays for different size sites.

		Figure 3: Traffic Chart for different capacity trunked sites

Unlike a simple conventional network, these talkgroups are very discreet, with only the people (who are authorised) in the talkgroup hearing the conversation.

Another way to compare a conventional with a trunked network is to think of a telephone network.  Imagine wanting to call Melbourne from Sydney, and randomly picking 1 of 1000 lines.  Maybe someone is talking, in which case you can either wait, or try a different line (or listen in!).  Not very private and a bit of a hit and miss approach.  This is similar to a conventional network.

A trunked network is similar to the way a telephone network handles calls to Melbourne.  The network picks a free line for you.  When you’ve finished your call, it is made available to the next person.  Your call is private, and not shared with other people you don’t want to talk to.

Further Reading

• EF Johnson's document regarding their LTR trunking systems, contains an easy-to-read "Introduction to Trunking" - The relevant section is provided here.