Responsive web design in a nutshell

Responsive web design is the presentation of web content in the most relevant format for the viewport and device accessing it.

In its infancy, it was typical for a responsive design to be built starting with the 'desktop', fixed-width design. Content was then reflowed, or removed so that the design worked on smaller screens. However, processes evolved and it became apparent that everything from design, to content and development, worked much better when working in the opposite direction; starting with smaller screens and working up.

Before we get into this, there are a couple of subjects I'd like to address before we continue; browser support and text editors/tooling.