A responsive website changes its appearance and layout based on the size of the screen the website is displayed on. Responsive sites can be designed to make the text on the page larger and easier to read on smaller screens. They can also be configured to make the buttons on the phone’s screen easier to press. More sophisticated ways of using responsive design on a mobile device include: formatting the website to hide or present entirely different information, radically changing the graphics and colors, or even reducing the site to emphasize just its most important piece.
You can test a site to see how it looks on each screen here.
An example of a fully responsive site and all the different views.