JavaScript is a scripting language that makes web pages interactive by dynamically updating and animating content in the browser. It was created by Brendan Eich at Netscape and allows developers to create rich, user-friendly experiences on the web. JavaScript differs from Java in that it runs in browsers rather than virtual machines and only creates applications, not full programs. NodeJS provides a runtime for JavaScript projects and uses npm to manage packages. ReactJS is a library that makes it easy to build interactive web pages, relying on NodeJS to execute code and requiring packages installed through npm.