CSS is the short form for Cascading Style Sheets. With CSS you can create more complex, and attractive documents than you can with ordinary HTML. You can use it to colour or capitalise text, to control text spacing or even to place borders around most elements on the page. Also, using CSS, you can create pages which will download much faster because they are leaner and slimmer than conventional HTML pages-the reason for this being that the CSS rules are written to an external file which is then linked to your pages. When your page is viewed in a web browser, the browser will then search for the external file and after loading said file, apply the rules contained therein to successfully render the HTML document.
You might think that because the browser still has to seek out and load this external file, that the web page would still take a while to load but in fact after the style sheet has been loaded for the first time, it is in the browser's cache and accordingly can be applied easily and quickly to any or even all your web pages. Also a typical HTML web page is very code heavy! That is to say that there are huge numbers of font tags and tables and various other code artifacts which are frequently repeated throughout the entire HTML document and these can increase the file size of an HTML page substantially. With CSS, better results can be achieved using much slimmer or more compact code thus causing the web page to download really much more quickly.
Another advantage of CSS is in updating your website for example: if you are needing to change all of your headers in your website which you have created as H2 elements in a brown colour and you have tens and tens of them in all your web pages. When you need to recolour all of them for example blue, all you have to do is make the change once in the style sheet and they will be applied instantly to all the H2 elements throughout your entire site.
One slight disadvantage with using CSS is that of Browser incompatibility.Some browsers do not support CSS as well as they might! There are discrepancies in the way the CSS enabled web page will display for example between IE6 and IE7 - the latter being much closer to Firefox in the way it renders CSS Web pages.