Trunks in telecommunication are links that carry many simultaneous calls; if there were no such things as trunk lines then a company would have to buy an telephone connection for every single extension that ran off its PBX. Instead, the Telecom company runs trunk lines to connect the PBX to the local Telecom exchange. In this way many extensions can share these few lines that make up the trunk line. In a standard PBX there will be cards for terminating analogue trunk lines that come in 12 or 24 port configurations. With analogue lines one call is carried per line.
There are several types of Digital ISDN Trunk lines starting with the smallest a 2-line ISDN basic rate line called a BRI. The BRI ISDN line is typically only used for very small offices with few telephone users. However BRI has an interesting feature in that it can send and receive at the same time so two channels can be open one for data and one for voice simultaneously, or there could be two simultaneous voice calls over each individual line.
For larger offices a telecom company will run a PRI trunk, which dependent on which part of the world your office is located will be either a 24 or a 30 channel trunk, providing he support for 24 or 30 concurrent calls through the office PBX.
In the USA and Japan a PRI ISDN trunk is called a T1 in Europe and the rest of the world an ISDN trunk is called an E1. The difference between a T1 and an E1 is down to the number of channels and therefore the size of bandwidth or call capacity. An E1 supports 32 channels however, 2 are utilized for signaling giving a total of 30 call bearing channels. A T1 has 24 channels and all can be used for bearing calls as signaling is not carried on a separate channel.
Although PRI ISDN trunks come in 24 or 30 line configurations telecom usually provide split configurations whereby a customer can by a subset of the lines available. For example, they may only require a maximum of 12 lines not 24.
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