The term Trunk derives from a structure of a tree where one trunk has many branches that it feeds with nutrients.
Similarly in in telecommunications or networking application, an infrastructure (the tree) has one trunk line that carries many voice and data signals simultaneously into many users(the branches). When talking about a trunk we are referring to a single transmission channel between two points, each point being two switching systems, such as the one in a telephone company central office or between PBX systems. Combine all those switching and PBX systems and you get your one tree. Combine all the trees and you have the entire communication network in the world.
SIP Trunk basically uses the SIP protocol technology to transform all or part of your broadband connection into communication trunk that functions like a virtual phone line (Of course if you have a large call volume you may want to get a dedicated broadband connection, just for your telephony needs).
PBX Trunking is the process in which your PBX system merges data from multiple calls into a single set of packets to reduce transmission overhead. Basically what this means is that many users share one broadband connection (or trunk) to make calls and this is plausible thanks to modern hardware and software that compress and combine the date from those calls. To illustrate this think of one hundred people that needs to get from point a to point b. Traditionally they would use one hundred cars (phone lines) to do so. But if we "trunk" them into one railway we we only need one high speed train (trunk). Therefore, as you can see, with existing infrastructure trunking is both more cost effective and more efficient.