XHTML & CSS Tutorial - What is XHTML & CSS?
XHTML stands for Extensible Hypertext Markup Language and is a language your internet browser like Internt Explorer or Firefox understands and uses to display webpages. It's intended to replace the older HTML because it's more stricter and clean.
CSS stands for Cascading Style Sheets and is used to style your websites. It was designed to sort out the big mess that HTML had become over the years. HTML was made to define parts of a website like "This is a paragraph" or "This is a picture" but over time the browers added new things like colour and fonts, this basically made it all a mess. So CSS was made to sort things out. They are normally saved in a .css file and linked to from a webpage. They can be linked to from any number of webpages meaning you only have to edit the one CSS file instead of hundreds of pages to change the font size by one or something thus saving you alot of time.