JavaScript is an essential programming language for web development, enabling the addition of dynamic and interactive features to web pages. It runs client-side in web browsers, allowing real-time manipulation of HTML content and CSS styles, managing user events, and creating responsive web applications. With JavaScript, developers can implement animations, form validations, API interactions, and much more, enhancing the user experience. In addition to client-side code, JavaScript is also used server-side with environments like Node.js, thereby extending its capabilities to create complete and scalable web applications.