If you're an old school web guy, you would have learned to design web pages in a grid-system using tables. This can be problematic for a number of reasons, the most basic of which is your website will load slower as a result, as although each page may contain the same design elements, the web browser has to download the design at every page.
Enter Cascading style Sheets (CSS). CSS seperates the display layer from the content layer of the website - in other words, you load the website's design once, and then only load new content at each pages.
If you want to learn CSS, check out this amazing css tutorial. If you want to do advanced CSS design, check out this tutorial on CSS Floats , or http://snook.ca/archives/html_and_css/six_keys_to_understanding_css_layo... .