CSS (Cascading Style Sheets) allows you to define how HTML elements are displayed. Styles are saved in external .css files so that the appearance and layout of all pages on a website can be changed by editing just one file. JavaScript adds interactivity to HTML pages and is embedded directly into web pages. It is a lightweight programming language that is interpreted as pages load rather than needing compilation. PHP is a server-side scripting language that is widely used and efficient. It allows for server-side validation and dynamic front-end display. PHP scripts are executed on the server and support many database types.