Back in the old days of HTML, most attributes applied to text, page design, and fonts had to be done so inside the HTML code. This made for messy coding and also made it very time consuming and tedious to do changes to a website. Changes having to made to a large website was especially difficult because of having to go into each individual page to things like color changes, changing font style, and font size changes.
Several years back Cascading Style Sheets, know as CSS, was introduced into the website design world. This revolutionized the way that designers make and edit web pages. While the roots of CSS date back to the early to mid 90's (around 1993-94), the wide-spread use of it did not really start showing much in websites until the late 90's and early 2000's. Of course it takes a little bit of time for technology to spread, although things spread much more faster today than they did fifteen years ago.
Modern day CSS is a combination of the work of several different individuals who knew their could be a more efficient way of designing and managing web pages. The main goal of CSS was and is to separate the actual content in a web page from the way it is shown in the browser. For example; font types, font colors, and font sizes are specified in style sheets instead of in the HTML (when CSS is used on a site).
The W3C maintains the rules and standards of modern CSS. Just like with any other computer language, there is a specific syntax and words that must be used in order for the style sheet to work correctly. The use of CSS has become increasingly popular through the years as more web developers and designers have discovered the advantages to using it. Once you learn how it can help you manage a website, you will never go back to the 'old-way' of designing sites. If you'd like to learn how to use style sheets, there are several books and better yet, online tutorials that can help teach you how to develop and apply it.