Node.js is an open-source, server-side JavaScript runtime environment that allows developers to run JavaScript on the server. It is built on the V8 JavaScript engine, which is the same engine used by Google Chrome. Node.js provides an event-driven, non-blocking I/O model that makes it lightweight and efficient for handling concurrent connections.

Traditionally, JavaScript was primarily used for client-side scripting in web browsers. However, with the introduction of Node.js, JavaScript can now be used on the server-side as well. This means that developers can write JavaScript code that runs directly on the server, enabling them to build web applications, APIs, and other networked services.

Node.js comes with a rich set of built-in modules that provide various functionalities, such as file system I/O, networking (HTTP, TCP, UDP), and more. Additionally, it has a vast ecosystem of third-party modules available through the npm (Node Package Manager) registry, which makes it easy to incorporate external libraries and frameworks into Node.js applications.

One of the key features of Node.js is its non-blocking, event-driven architecture. This means that Node.js applications can handle multiple simultaneous requests without getting blocked, allowing for highly scalable and efficient applications. It achieves this through the use of callbacks, promises, and asynchronous programming techniques.

Overall, Node.js has gained popularity among developers due to its ability to unify front-end and back-end development using a single programming language—JavaScript. It has become widely used for building web servers, real-time applications, microservices, and even desktop applications.

Node.js is widely used for web development, creating APIs, building server-side applications, and working with databases. It has gained popularity due to its ability to leverage JavaScript skills on both the front-end and back-end, enabling developers to work with a unified language and codebase throughout the entire application stack.