CSS3 media queries enable us to target particular CSS styles to particular device capabilities or situations. If you head over to the W3C specification of the CSS3 media query module (http://www.w3.org/TR/css3-mediaqueries/), you'll see that this is their official introduction to what media queries are all about:
"A media query consists of a media type and zero or more expressions that check for the conditions of particular media features. Among the media features that can be used in media queries are 'width', 'height', and 'color'. By using media queries, presentations can be tailored to a specific range of output devices without changing the content itself."
Without media queries we would be unable to substantially alter the visuals of a website using CSS alone. They facilitate us writing defensive CSS rules that pre-empt such eventualities as portrait screen orientation, small or large viewport dimensions, and more.
Whilst a fluid...