Continuing on with some telecom basics, here's a basic overview of trunking.
A trunk is a single channel that allows data to move between two points. If you have a network topology map handy, you could consider the lines between each switch to be a trunk, though it's slightly different in telephony.
Trunking is a way to send multiple conversations over a single line. This is highly related to my previous PBX article where an organization rents a couple of lines for use by all members.
On a very abstract level, every time you connect a device to your computer, like an external drive or mobile device, you could consider the cable to be a trunk. Point A is the external drive. Point B is the computer. The cable between the two is the trunk.
Now, if you add a USB hub to the mix, you have an example of trunking. Let's say you have an external drive and a mobile device. You could plug both into your computer, but maybe you only have one open port. So you plug in the USB hub and plug both the external drive and mobile device into the hub. They both send signals through the cable connecting the hub to the computer. The act of both devices sending signals through the last cable from the USB hub to the computer (and not getting the signals mixed up) is trunking.
More realistically, a trunk is the massive bundle of lines your telephone company controls. Organizations rent a few lines from the telephone company, generally called trunk lines. Rented lines are expensive, so organizations generally try to rent as few as possible. The trunk line goes to a PBX system (or IP PBX systems nowadays), and organizations use their PBX systems to coordinate which device gets what information and then decide which lines transmits the data.
Trunking comes in a few different formats as well, such as SIP and T1 PRI. This is controlled by the PBX system and your telecom provider. Stay tuned for more information on SIP and PRI trunking.