CSS = Cascading Style Sheets and is the presentation layer for web pages. That is how you format text, divide up your content and give different displays to sections of your page. Using this forum for example We have a graphic that is a background image to forum section. The arrow at the top right expands or contracts a block of infromation about the forum (as do the + signs you see on other parts of some of the pages) which is done using a combination of JavasScript & CSS.
Using CSS the default display of the forum title was changed and moved so that it would not overlap the border that is part of the masthead background image. This page uses a font that is not the default on any web browser I've ever seen. It does so because the default for the body element was redefined using CSS. CSS is what keeps your web page from looking like the outlines you created in your early schooldays.
