Fluid Websites are designed to work on all devices. It is website industry best practice for improved SEO.
A Responsive Website is usually written in HTML and styled with CSS to automaticaly resize and fit perfectly on any given screen size of a web browser. Responsive Websites look good on all devices big or small (desktops to mobiles), in order to offer the same great user experience. This means, one URL with the same HTML markup is crawled and indexed twice by search engine desktop bots and mobile bots, thereby improving SEO rankings.
That's good Web Design.