The basic concept of CSS is that it should separate information from presentation. While HTML, XHTML or XML should take care of the information, CSS should take care of the presentation.
HTML was originally only designed to handle information, but page authors required more control over the look of documents therefore tags such as <font> were introduced to satisfy this.
This blurred the division between information and presentation therefore CSS was introduced. This allows different styles to be applied to a document, or group of documents, quite simply.
The Advantages of CSS
Style definitions can be stored in a separate file and applied to a group of documents. Changing the style of the documents means only a change to the style sheet file.
Style sheets give much greater control over the appearance of documents than previously possible.
The separation of style from information allows smaller file sizes since style information isn't carried within each document - only information.
Why You Should Learn CSS
If you have any doubts about learning CSS consider the following. Imagine you have developed a 100 page website. Throughout your pages you have highlighted certain areas of text in red by using the font element thus.
<font color="#ff0000">Highlighted text.</font>
You decide you'd like to change the highlight colour to green. This means ploughing through every page, finding every occurrence of font elements set to red and changing them to green.
If you had used CSS then each occurrence of highlighted text might be as below.
<span class="highlight">Highlighted text.</span>
A span tag has been used, instead of a font tag, and has been set to a class called "highlight" which can be set to whatever style you require within a style definition.
To change the appearance of "highlight" means only one change to the style definition and the appearance of "highlight" will change throughout the entire site.
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