For advanced reading about monitor controllers, Chris Muth had great influence to how I understand audio monitoring today. Before I experienced the differences of various controllers, monitoring was about a monitor section of a console, some cable and a few sets of speakers.

This is not an advertisement for Dangerous Music. Its merely a topic about audio monitoring and what I found helpful for me.
There are many ways to monitor sound and all sorts of monitor controllers on the market. This particular link helped demystify a lot of the questions I had. Hope it helps you and also inspires continued discussion on monitoring. :)
http://dangerousmusic.com/tutorials/monitoring/