Responsive Design creates a good user experience by displaying and working well on the different screen sizes of phones, tablets, laptops, desktops and televisions. This means designing a suitable site to adapt to the user's device, sometimes even rearranging or resizing parts of pages. Examples of such features include having a hideable navigation bar at the top of the page for smartphone users, or larger touchable links for tablet screens, or images that scale in size on different screens.
A poor design can be totally unusable, meaning people never get to the site in the first place due to poor positioning in search results, or they leave as it loads too slowly, or they give up in frustration as navigation and search on the site turn out to be a hopeless labyrinth. Even a badly done responsive design can lead to maddening scrolling on mobile devices and menus that make no sense.
Good responsive design delivers a consistent appearance across devices, and image size is also a key concern. Visually speaking, images need to scale to fit the screen, but also images which are relatively large files and work fine on broadband may be just too big to load in a reasonable time, or load at all, on slower mobile networks. So effective Responsive Design is about more than just appearance, it's also about strategy and testing too.