CSS is used to style and lay out web pages. CSS3 is the latest standard. CSS rules contain selectors that target elements using properties and values to style them. There are block level elements like headings and paragraphs and inline elements like bold and images. Important selectors include universal, ID, class, descendant, type, and pseudo-class selectors. CSS can be embedded within HTML using the style element or linked externally using the link element to reference a .css file. Attribute selectors allow targeting elements based on attribute values that begin, end, or contain certain strings.