Could someone please explain to me, in simple terms, the difference/relationship between Responsive Site Designer and Responsive Layout Maker.

With RLM, which was created first, you can create a layout with no real content, just place holders. Great for planning a site. The idea is that you make the layout (or wireframe), and then export it so that you can add contents in a normal web editor.

With RSD you can create both the wireframe and the contents, in other words, you can make a complete website. If you have wireframes from RLM, they can also be imported into RSD and developed further. You cannot go the other way, though, open a RSD-made site in RLM.