Fixed Layouts use the unit pixel (px). This type of layout neither adjusts to the viewport (visible area of browser) nor to the default font size in web browser.
Flexible Layouts use percent (%) as unit. This type of layout adjusts automatically to the viewport respectively to users screen resolution.
Liquid Layouts are based on the units "em" or "ex". Such layouts grow and shrink everything when the user changes the text size (other layouts only modify the text size).
Fixed font sizes are a problem in many browsers (for example, IE6) because the user often cannot change the settings. For accessibility it is therefore a must to use relative units (em).