Answer 1

The main difference is that Fluid Layouts (also called Liquid Layouts) are based on proportionally laying out your website so elements take up the same percent of space on different screen sizes, while Responsive Design uses CSS Media Queries to present different layouts based on screen sizes/type of screen. For some examples of both kinds of design, see Inspiration: Fluid & Responsive Design.

Fluid's intent is to keep the same spatial weighting to all elements, and works okay on different sizes of screens of the same sort. They tend to look okay on a 32", high resolution monitor and a 12" lower resolution laptop. They're pretty easy to implement.

Responsive design's intent is to serve different devices layouts tailored specifically for the type of screen. Your site's layout will generally be cut down to a single column on a smartphone for example.

For a deeper look, check out Fixed vs. Fluid vs. Elastic Layout: What’s The Right One For You? by Smashing Magazine, which goes in depth on the primary changes in Fluid design. They also have Responsive Web Design Guidelines and Tutorials for an overview of responsive (with plenty of links should you want to dig deeper into Responsive Design).

Responsive Design is very much "in" at the moment thanks to Mobile, Smashing actually has a whole tag for responsive design.

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