CSS stands for Cascading Style Sheets, which is a styling language that sets rules to determine how a webpage should look. There are three ways to write CSS rules: inline within HTML elements, internally within <style> tags in the head section, or externally in a separate .css file linked via <link> tags. CSS rules have selectors that target elements, and declarations with properties and values that set styles like color, font, and layout. For example, p {color: yellow;} would make all paragraph text yellow.