Courcelle's Theorem is an important result in graph theory, proving the existence of linear-time algorithms for many decision problems on graphs whose tree-width is bounded by a constant. The purpose of this text is twofold: to provide an explanation and step-by-step proof of Courcelle's Theorem as applied to graphs of tree-width bounded by a constant, and to show explicitly (on the example of path-width) how to apply the same principles to other graph classes. We present these topics in a way that does not assume any particular knowledge on the part of the reader except a basic understanding of mathematics and possibly the fundamentals of graph theory. Our hope is to make the topic accessible to a broader mathematical audience, to which end we have included extensive explanations and pretty pictures.