CSS

CSS stands for Cascading Style Sheets, they define how to display HTML elements
External Style Sheets can save you a lot of work
External Style Sheets are stored in CSS files
Multiple style definitions will cascade into one

Styles Solve a Common Problem
HTML tags were originally designed to define the content of a document. They were supposed to say "This is a header", "This is a paragraph", "This is a table", by using tags like <h1>, <p>, <table>, and so on. The layout of the document was supposed to be taken care of by the browser, without using any formatting tags.

As the two major browsers - Netscape and Internet Explorer - continued to add new HTML tags and attributes (like the <font> tag and the color attribute) to the original HTML specification, it became more and more difficult to create Web sites where the content of HTML documents was clearly separated from the document's presentation layout.

To solve this problem, the World Wide Web Consortium (W3C) - the non profit, standard setting consortium responsible for standardizing HTML - created STYLES in addition to HTML 4.0.

Both Netscape 4.0 and Internet Explorer 4.0 support Cascading Style Sheets.
From W3 Schools, Read more at: W3Schools CSS Tutorial