CSS is the acronym for "Cascading Style Sheet". CSS is a stylesheet language that describes the presentation of an HTML (or XML) document.
CSS describes how elements must be rendered on screen, on paper, or in other media.
With CSS you define the colors and sizes in "styles". Then as you write your documents you refer to the styles. Therefore: if you change a certain style it will change the look of your entire site.
Another big advantage is that CSS offers much more detailed attributes than plain HTML for defining the look and feel of your site.
Finally, CSS can be written so the user will only need to download it once - in the external style sheet document. When surfing the rest of your site the CSS will be cached on the users computer, and therefore speed up the loading time.