The following is the first paragraph of the Introductory chapter: What is Fictional Cartography of the the 25 Quick & Dirty Map Tutorials Guide scheduled for release in August 2014, by me.
Cartography is the art and science of visually depicting the world around us in graphic format as maps. It is an ancient activity with examples as old as civilization itself, though a more contiguous history can be followed over the last 2000 years. Cartography uses mathematics, measurements and symbolic representations of landforms to represent our world in two dimensional format – as one or more printed sheets or displays on our electronic devices. While cartography is very much a science, this guide focuses on the art of cartography to a greater degree, and the techniques used to depict aesthetically pleasing results that help players imagine the fictional worlds they explore in the course of a game or other work of fiction. Creating maps for the purpose of depicting fictional places is the realm of fictional cartography and the focus of this guide.
I think it close to the descriptions in RobA's post.