CSS (Cascading Style Sheet) is a style sheet language that determines the look of elements on a web page. Web sites can be built without CSS (it came many years after HTML), but CSS gives the web designer more power to control how the content is displayed. The main benefit of CSS is that it splits the content from the presentation.
CSS allows for more accurate control over the way a web page is rendered. CSS allows designers to create custom styles that are then applied to the web site in one of a variety of ways. The main benefit is that something like text colours for an entire site can be changed by editing only the CSS file. CSS can also be used in SEO, but most SEO techniques that involve CSS are considered spam.
Site-wide style changes can be performed by editing the style sheet rather than every page individually.