Of course I am able to answer this question myself; but I have several other self-answered questions in the works, and I want to help set the tone for missing, important reference questions like I have been talking about a bit in Meta. I expect that having this (and similar efforts) as a high quality Q&A will make it easier to answer other beginner questions in the future, since they can link here instead of re-explaining the concept in the context of another answer.

... Although, as I read these answers, I feel more and more like the concept of a statement is something we usually understand intuitively, and which is much harder to explain than it initially appears. (Expressions still seem fairly straightforward, though.)