The Key Concept:

Cascading style sheets, or CSS have been promoted by the W3C as an alternative to HTML tags such as <font> that have been used to format text, and to control some elements of page layout. Those tags are now "deprecated," that is, the W3C would prefer that Web designers no longer use them.

Instead, they are promoting the use of CSS. The main concept to understand with CSS is this:

CSS separates content from structure.

Mouse over the light bulb. Think of the light bulb as a way to remember the "content" part of this principle.