Grid systems have been used in print design, architecture and interior design for generations. Now, on the web, the same rules of grid system composition and usage no longer apply. Content is viewed in many ways; from RSS feeds to email. Content is viewed on many devices; from mobile phones to laptops. Users can manipulate the browser, they can remove content, resize the canvas, resize the typefaces. A designer is no longer in control of this presentation. So where do grid systems fit in to all that?
http://www.webdirections.org/resources/mark-boulton-designing-grid-systems/