What Is CSS
CSS (Cascading Style Sheets) is a simple way of controlling the style & formatting of a Web page. By separating the visual design elements (fonts, colors, margins, etc.) from the structure of a Web page, CSS gives Web Designers the control they want without sacrificing the integrity of the data & structure of the site. Also, you can define typographic designs and page layouts from within a single block of css code - without having to resort to image maps, <font> tags, tables, (if you do use tables, you can apply CSS to that as well) and spacer GIFs, allowing for faster download times, streamlined site maintenance, and instantaneous global control of design elements across a whole site.

How CSS Works
CSS overrides the browser's default settings for interpreting how tags should be displayed, letting you use any HTML element indicated by an opening and closing tag (including the <p> tag) to apply style attributes defined either locally or in a stylesheet.

Stylesheets contain rules, composed of selectors and declarations that define how styles will be applied. The selector (a redefined HTML element, class name, or ID name) is the link between the HTML document and the style. There are two different kinds of selectors: types (HTML element tags) and attributes (such as class and ID names).

A CSS declaration has two parts, a property ("color") and a value ("red").

The basic syntax of a rule:
selector {property 1: value 1; property 2: value: 2}

An example (containing two declarations, as above)
P {font-size: 8pt; color: red}