The concept behind CSS (a.k.a. cascading style sheets or style sheets) is really simple. CSS allows
Website design services to make changes to all of the web pages that link to the CSS file at once by changing a style in the style sheet, instead of having to manually change every style in every HTML file.
CSS allows you to create a single document of code, similar to an HTML file that lets them specify the colors, fonts, backgrounds, etc. of a web page. The CSS file is then linked to from the web page(s) that they want to have the same styles that they specify. If CSS did this and only this, they would save their lot of time to say the least, especially if they have a large or multiple web sites. This alone is worth learning CSS; however, style sheets allow them to do this and much more.
CSS also allows them to: position text and graphics precisely where you want to, add rollover effects to links, control the spacing between letters, lines, margins, web page borders, specify the units such as centimeters, pixels, points and more and hide content from certain web browsers in certain situations. An example of this is when you have some content that you want to appear only in your web pages, but not in print. In the end, CSS can save you a lot of time and effort and is very easy to learn.