JavaScript is a high-level, versatile, and widely-used programming language primarily known for adding interactivity and dynamic content to websites, making them more engaging and user-friendly. It runs on the client-side, in the web browser, allowing for real-time updates and interactive user interfaces. Alongside HTML and CSS, JavaScript is one of the three core technologies of the World Wide Web. It has evolved over time and is now used on the server-side as well, thanks to environments like Node.js.