CSS stands for Cascading Style Sheets.
Nowadays there are hardly any sites build without CSS anymore. CSS can make your "styling" of the site quite easy. What's done most with it is the coloring of text, headings, the decoration of links, even the complete layout can be done with CSS. These pages all use an external style sheet for the layout and the sizing and color of the text. Also the neat changes you see when you hover over the links, they are ruled by the CSS style sheet.

All there is to it, is that you have to remember that, the CSS syntax is made up of three parts: a selector, a property and a value. The selector is normally the HTML element/tag you wish to define, the property is the attribute you wish to change, and each property can take a value.

notice the curly brackets around the property and value, also that there is a colon between them.

When you want to put more then 1 property into a tag, you separate them with semi-colon.