Although this video was created using AutoCAD 2008, the same commands and techniques apply to newer versions. View the tutorial that relates to this video.
Dynamic blocks are using in AutoCAD to give your blocks more versatility. You can add parameters to the block that allow you to stretch, rotate or even change the appearance of the block. Think of one block that can have 6 different types of sinks, each a different size.
Dynamic blocks are a great time saver.