A Few Words about CSS

CSS is a separate from HTML language. It is used for defining styling, positioning and some other behaviors of HTML elements across ALL pages on the site where you do not need to repeat such "styling" for each HTML element, like font and it's size but rather simply refer to the one place determining the way all chosen elements look and behave (to a certain extent). Complex behaviors require JavaScript but a LOT could be done using CSS definitions. CSS, and CSS3, as its last incarnation, is continuously developing assuming more and more power under the current course on HTML5 leadership in its trio of HTML, CSS, and JavaScript being the core  of these technologies.

The current trend is in giving HTML the role of the architectural and semantic tool in developing web resources (like web pages) where the HTML5 compliant browsers are the readers and interpreters of this information. Special semantic processors and search engines are coming in the near future to benefit from such clear structural/semantic role of HTML where they will be able to parse the HTML text and find the role, meaning, and relationship of various terms and sentences and understand them according to HTML blocks marked as "header," "section," "aside," "footer," "figure," "figcaption," etc.
CSS is supposed to separate content from presentation features that might be different for different users showing more or less and in different ways by just using different CSS files connected to the current HTML. This allows the separation of content and presentation/access management. Same goes for external JavaScript files that could be changed for different users and circumstances or programmed with lots of conditional IF statements changing behavior in relationship with different circumstances.

For simple general overview of CSS - you might see  Introduction (along with many other sources that you will find in your research on CSS). The list of new features can show you the way CSS3 is being enhanced in the current CSS3 info. An additional to w3schools example of some text-handling capabilities can be seen here.
I encourage you to read more about CSS and experiment with w3schools' examples as well as with examples in other sources. The more research and experimentation you put in this program - the more you get from it.