Responsive desing. That's an easy one and probably most people know what that means and even take it for granted. To really understand though, you should have at least two devices with differnt screen sizes. Say you have an iPhone and a MacBook, just so that we stick to Apple :) You must have noticed that when you visit the same website on these devices they look just different, both filling nicely the shape of the device.
This adapting to the device's shape is what they call responsiveness. The design and arrangement, size of the elements on the page responses to the size and shape of the screen. Responsive design has become a standard in the last 10 years or so, with the exponential spread of smart phones it is now a must, making developers to come with ingenious ways of solving sometimes difficult design situations.