Traditional layouts are primarily used for desktop applications. Though the popularity of non-fixed layouts became more widely used as various desktops, laptops and tablets came into play with varying screen sizes, they are not truly adaptable to the smaller, hand-held devices such as smartphones.
True responsive design is derived from the standpoint of dynamic size dimensions. For example, element sizes are based on percentages, scalable vector graphics are used which display with the same precision regardless of size, blocks fall from rows into columns of varying dimension based on viewport. Responsive design includes browser detection and can accommodate multiple viewports.
Mobile is the wave of the future and it will be around a while.