What Is CSS?
"CSS" stands for "cascading style sheets." CSS relies on an underlying HTML structure and so you cannot create a Web page purely out of CSS. CSS consists of rules for how HTML elements should behave, and so if there is no element for a style to control, the style is meaningless. A simple real-life example would be a game, say, hide-and-seek. The rules (the CSS) are that one person closes their eyes and counts while the other person hides. If there are no people playing (the HTML elements), the rule is pointless.
What is the 'C' in CSS?
Great! Now that we know that these styles are rules, what's the cascading part? The cascading concept for style sheets refers to the precedence of (in increasing importance) external, internal, and inline styles. Basically, the cascading concept means that more than one style can affect the same element and multiple rules can apply to multiple elements. It can get really confusing if you don't keep track of the rules, but we'll keep it simple for now.
What is CSS REALLY!?!
If I hasn't sunk in yet, I'll give you one more powerful example. If your site is interesting (and we all know it's interesting) and contains multiple colors, different margins for certain categories of information, and an array of different font sizes among all the other features of your site, what would the coding for that be like if you're restricted to pure HTML? You won't just create a site and leave it alone, so how will you ever change all those size 7, Verdana font, bright red page headings to size 3, Arial font, green headings before all the hair on your head falls out? The answer is simple and its name is CSS. Create a single rule for "page headings" and apply it throughout your site. When you tire of the way the site looks or if you want to update it, just change that the way that "page headings" are defined, and the change will ripple through your entire site.
I still don't get CSS!
I'll relate the CSS concept and you in an aggressively, yet positively self-confidence-building manner at the same time: You will have ultimate control over the layout and look of your site! sinister voice>Bwaaa Ha ha ha har har har har!!!/sinister voice>