While HTML is used to describe the content in a web page, it is Cascading Style Sheets (CSS) that describe how that content should look. In the web design biz, the way the page looks is known as its presentation. That means fonts, colors, background images, line spacing, page layout, and so on… all controlled with CSS. With the newest version (CSS3), you can even add special effects and basic animation to your page.
CSS also provides methods for controlling how documents will be presented in contexts other than the traditional desktop browser, such as in print and or on devices with small screen widths. It also has rules for specifying the nonvisual presentation of documents, such as how they will sound when read by a screen reader (although those are not well supported). Style sheets are also a great tool for automating production because you can change the way an element looks across all the pages in your site by editing a single style sheet document. Style sheets are supported to some degree by all modern browsers.
Although it is possible to publish web pages using HTML alone, you’ll probably want to take on style sheets so you’re not stuck with the browser’s default styles. If you’re looking into designing websites professionally, proficiency at style sheets is mandatory.