Cascading style sheets is a language intended to simplify website design and development. Put simply, CSS handles the look and feel of a web page. With CSS, you can control the color of text, the style of fonts, the spacing between paragraphs, how columns are sized and laid out, what background images or colors are used, as well as a variety of other visual effects.
HTML and XHTML are very similar languages. In fact, for the majority of documents today, they are pretty much identical, although XHTML has some strict requirements about the type of syntax used.