Jeremy Keith has added his few pence to the discussion. He starts with the three parts that Ethan Marcotte identifies as key pieces of responsive web design:
- A flexible grid.
- Flexible images. Or more specifically, images that work in a flexible context (whether fluid themselves, or perhaps controlled via overflow).
- Media queries. The final layer of a responsive design, media queries optimize the design for different viewing contexts, and that spot-fix bugs that occur at different resolution ranges.
A good read and I encourage you to check out Ethan’s more in-depth clarification about it.