CSS (Cascading Style Sheets) is a style sheet language used to describe the presentation of structured documents written in HTML or XML. CSS was introduced to separate document content from presentation, allowing the same content to be styled differently. CSS syntax uses selectors to target elements and declaration blocks with properties and values to style them. The "cascading" refers to how CSS prioritizes styles from different sources using a specificity scheme. Future developments include new selectors, blend modes, gradients, and using HTML elements as backgrounds.