The Idea Behind CSS
Cascading Style Sheets (CSS) is a stylesheet language used for managing the look and formatting of a website.
The idea behind CSS is to separate the content of a website from its design. It includes elements like layouts, fonts and colors. This separation will save you a lot of time when you need to change the outlook of your website and keep the existing content. Once the CSS file is included in your pages, you will no longer have to edit the design of each new element you add to your pages - the CSS will take care of that.
CSS makes it easier to allow different people with no web design knowledge to add content to your website. Popular CMS systems like Joomla, Drupal or WordPress wouldn't be the same without CSS.
The CSS files provide you with a lot of flexibility. Imagine that after you add all of your content, you want to change the color of your text or its font. If you use a CSS file, you will have to change only a single line. If the styling of your elements is added directly to the HTML code, you would have to make hundreds and even thousands changes. CSS allows you to make global changes as well as unique modifications to single parts of your website.
In addition, CSS files save you bandwidth and reduce the loading time of your website. If you specify the looks of each element in your website directly, without using a CSS file, you will have to add much more tags to your code which will make the file much bigger and heavier to process.