Defining responsive web design
The term, "responsive web design" was coined by Ethan Marcotte in 2010. In his seminal A List Apart article (http://www.alistapart.com/articles/responsive-web-design/), he consolidated three existing techniques (flexible grid layout, flexible images/media, and media queries) into one unified approach and named it responsive web design.
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.