CSS, which stands for Cascading Style Sheets, is a language used alongside HTML to separate a webpage's content (the HTML part) from the design and formatting (the CSS part). It defines fonts, colors, margins, lines, height, width, backgrounds, images, positions and other visual aspects of a webpage, something which HTML was not originally designed to do. Simply put, CSS decides how your HTML is displayed.
CSS is supported by all modern browsers today, however each browser (and each version of each browser) may interpret CSS in different ways. It is always best to view your documents in multiple browsers to check their appearance in each one.
CSS can be applied to an HTML document using any one of three methods:
Inline: CSS is added directly to an HTML element.
Internal: CSS is added to an HTML file's header and affects the elements in that document.
External: CSS is maintained in an external file containing only CSS and referenced by the webpages on your website.
CSS is a very powerful tool, as it affects the mood and tone of your entire website with the benefit of less code and more options than HTML.