What is the difference between responsive and fixed width layouts?
Fixed width layouts are just that, fixed. Whatever the resolution of your display, the website will always be a fixed width. If the visitors display size is smaller than the fixed width the website will not resize dynamically. Viewports are used to shrink the site on small displays, like iPads and iPhones, but it does mean the visitor has to zoom in to read text.
Responsive layouts will resize to any display size. On a large desktop display the website can be full screen, or set to a max width. As the display gets smaller the website will resize all the way down to a mobile phone display. This means that whatever size display your visitor visits on, the website is perfectly tailored to their display. The benefit of this is that text remains at a readable size on any device allowing easy access on any device.