...separating style from content...
Semantic Code (xhtml*)
It is no longer acceptable that web pages are built from chunks of images chopped out of a Photoshop design file and positioned by use of table cells - a method by which screen readers cannot interpret information in the correct order.
xhtml
css
finished design
*extensible hypertext markup language
CSS
Webpages are now generally built in such a way that a page of 'content' can be written out in a linear fashion like a book, and the design applied to that by a separate 'stylesheet' using CSS* - a means of controlling the typography, images and layout independantly of the page. This results in faster page loading as common elements are 'cached' and so do not need reloading by the browser.
A separate CSS file can be used to customise a web page for print - formatting it differently and removing unwanted areas (eg navigation, background images) to create a printer friendly) page.