JavaScript (JS) is a versatile programming language used for adding interactivity to websites and web applications. It enables dynamic content, such as animations, form validations, and real-time updates. JavaScript can be executed on both the client side and server side, making it a key technology for full-stack development. It is supported by all major web browsers and works seamlessly with HTML and CSS. With frameworks like React, Angular, and Vue.js, JavaScript powers modern web development.