CSS stands for Cascading Style Sheets and is a way for Web developers to define the look and feel of their Web pages while at the same time ensuring consistency. An addition to HTML in web development, it is used to define design and layout, including fonts, colours, margins, lines, height, width, background images, advanced positions and more.
CSS is a very powerful tool for a Web designer to learn because with it the entire mood and tone of a Web site can be easily set and adjusted. Well written style sheets can be updated quickly and allow sites to change what is prioritized or valued without any changes to the underlying code.