Responsive Web Design (RWD) is a relatively recent approach to building websites, that allows them to be viewed on any screen size to user is visiting the site on.
Media Queries are used to change the styles of a website at various ‘breakpoints’. The media queries can be set to any size, to change the styles whenever an element need to be restyled. Responsive websites can be built to certain widths and heights, between certain widths and heights, landscape viewports, portrait viewports, or a combination of these options.
Google announced Mobilegeddon in 2015, an update to their search algorithm which favoured websites which were responsive, making them appear higher in search listings.
I build fully responsive websites, meaning that websites fit the screen perfectly regardless of the device orientation, width, or height.