Responsive design uses media queries to detect the client resolution and adjust to fill the display accordingly.
Ethan Marcotte introduced the concept of responsive design, whereby you craft a site using technologies that allow the site to adapt and respond to whatever device renders it. Responsive design embraces all devices and displays as part of a single user experience. The goal is to design for the entire user experience, not for disconnected designs aimed at different resolutions.
How does one design for this entire experience? Media queries.
Introduced in the CSS3 specification, media queries give designers the ability to modify how their sites display across resolutions, whether a small BlackBerry, a large 24” desktop monitor, a portrait view or a landscape view.