The difference between Responsive and Adaptive Web Design.
Both concepts allow websites to appear lined up to the device you are watching them on. Designers / developers using these concepts allows them to stick with one set of programming code, regardless of the device. This makes maintenance of software systems cheaper and the user experience more rewarding. Schumacher Partners are using both concepts successfully in our web application projects for clients.
Read thiis article for a good explanation of both concepts to non-developers / designers: