Node.js is a runtime environment that enables the execution of JavaScript code on the server-side. JavaScript is the underlying programming language used within Node.js, and it's also commonly employed for client-side scripting in web browsers. Node.js is not a framework or a language itself; rather, it provides the tools and libraries for server-side development. Frameworks like Express.js are often built on top of Node.js to simplify and structure web application development, but Node.js itself is a versatile runtime environment that can be used for a wide range of server-side tasks beyond web development.

Node.js and Java are two distinct technologies used for server-side development, each with its own set of characteristics. Node.js employs JavaScript as its primary language and offers an event-driven, single-threaded model, making it efficient for I/O-bound tasks. In contrast, Java uses its own statically typed language and runs applications within the Java Virtual Machine (JVM), providing platform independence and supporting multi-threading for CPU-intensive tasks. Node.js is known for its simplicity and is often chosen for web applications, while Java is renowned for its performance, making it a preferred choice for enterprise-level software and large-scale applications. The choice between them hinges on your project's specific requirements, including performance, ecosystem, and development team expertise.