C.S.S. stands for Cascading Style Sheets; documents which contain styling rules for applying to HTML (or XML, along with a few other structural formats). Multiple style rules can point to one HTML element, in which case there needs to be a way to determine which rule should take effect. The term cascading describes the process of filtering down from general rules, until the most specific rule is encountered. That rule is then chosen to do its work..
Through CSS, presentation can be separated from structure. It solves the need for mashing styling rules in amongst HTML, like this:
CSS is read by web browsers. They take the markup document, then they apply the styling rules to the elements within that document.
