CSS comprises style rules that are made up of selectors, properties, and values. Selectors target HTML elements to which styles can be applied. Properties are attributes of HTML elements that can be modified by CSS, like color or border. Values are assigned to properties, such as assigning the color red to the color property. Common selectors include type selectors that target elements by tag name, class selectors that target elements by class attribute, and ID selectors that target unique elements by ID. Styles can be applied to single or multiple elements using CSS syntax.