In a recent post, Ethan Marcotte clarifies his definition of “Responsive Web Design”:

A responsive design is composed of three distinct parts:

1. A flexible grid.
2. Flexible images. Or more specifically, images that work in a flexible context (whether fluid themselves, or perhaps controlled via overflow).
3. Media queries. The final layer of a responsive design, media queries optimize the design for different viewing contexts, and spot-fix bugs that occur at different resolution ranges.

See Ethan’s post, On being “responsive” — Unstoppable Robot Ninja.