What's the difference between adaptive, responsive web design?

Responsive web design is where your web page effective "responds" to the available screen space. Using primarily HTML and CSS we can "fluidly" adjust the width of elements and even hide elements to fit the available space. You can usually see the "response" by simply changing your browser window size.

Adaptive web design is a technique where you adjust the interface to suit the viewer's capabilities. This can be looked at as a set or rules implemented based on the viewing device's capabilities, including screen size. Often using a combination of technologies such as HTML, CSS and JavaScript the programmer fires certain rules for certain devices. Elements may be resized, moved, replaced, removed or even show a completely new interface. You may never know that a website is designed using the adaptive technique unless you view it on several different devices.