Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. It allows developers to run JavaScript on the server-side, as well as on the client-side. Some of the key features and concepts of Node.js include:

Non-blocking I/O: Node.js uses an event-driven, non-blocking I/O model, which makes it well-suited for building scalable and high-performance network applications.

Modules: Node.js uses a module system, similar to that of CommonJS, which allows developers to organize their code into reusable modules. Node.js also has a built-in require() function that allows developers to import and use modules in their application.

Package manager: Node.js comes with a package manager called npm (Node Package Manager), which allows developers to easily install and manage dependencies for their application.

Event Emitter: Node.js provides an EventEmitter class that allows developers to create and work with event-driven systems. This can be used to handle events like user interactions, network requests, and other asynchronous operations.

Express: Express is a popular web framework for Node.js that makes it easy to create web applications and APIs. It provides a simple and minimalistic API, and can be used to handle routing, middleware, and more.

Streams: Node.js provides a powerful streams API that allows developers to work with large amounts of data, such as file uploads and downloads, without loading the entire file into memory.