Cascading Style Sheets (CSS) are used to define things that were previously only able to be defined in HTML such as the layout, text size and font, table sizes, etc.
Web developers use CSS to create uniformity across website pages. Once CSS are installed across a site a web developer would be able to make uniform changes at the same time, like changing the size of a box on 20 different pages all at once.
This creates a high level of efficiency in saving time and creating professional looking pages. Style can be changed in the same way using CSS including color schemes. CSS gives web developers a greater control over their work than was previously available using HTML and it is replacing its usage in common development.
The web pages that have CSS will be defined to operate in a set order, sometimes one way in one situation and a different way under a different situation, depending on whether there is conflict within any elements.