It's called a liquid or elastic layout. Basically use a column layout that is defined by percents rather than fixed widths. For example, you could lay your page out like this:
LEFT COLUMN: 25% CENTER COLUMN: 50% RIGHT COLUMN: 25%
It's as simple as setting your CSS width styles to percent values, like this: width: 25%.
You can combine fixed and percent-based widths, as well, for a flexible layout.
Fluid (percentage based), elastic (em based) and fixed (pixel based) sites all have their plusses and minuses. Deciding which one is appropriate for your particular project depends on the amount of content you need to display per page, target audience, devices, and personal preferences.
Liquid layouts can get unwieldy on super wide or super narrow displays. Thus, if I have a lot of content to display, I prefer to use a semi-liquid layout - % based along with min- and max-width restrictions.
Elastic sites resize text and images proportionally which some people really like. Personally I prefer being able to resize text only in my browser without it affecting page and image dimensions. That said, elastic layouts can work well for graphics heavy web sites.
Fixed-width, centered layouts IMO are the most convenient to work with. I think that's why so many web designers use them. The trick is finding the "right width" for your target audience. For most desktop and laptop users, 900 to 1100px is probably a good range. For mobile devices, 480px.
Open Source CSS Grids, Frameworks and Multi-Column Layouts
Google labs - Browser Size