Node.js is an open-source, cross-platform JavaScript runtime environment built on Chrome's V8 engine, designed for building high-performance network applications. It uses an event-driven, non-blocking I/O model, capable of handling a large number of concurrent connections efficiently.

Main features include:

• Event-driven and Non-blocking I/O: Node.js uses an event-driven, non-blocking I/O model, making it suitable for building high-performance network applications.
• Cross-platform Support: Runs on Windows, Linux, and macOS.
• Rich Ecosystem: Boasts a vast npm (Node Package Manager) ecosystem with a large number of third-party modules.
• Easy to Learn: Easy to get started for developers familiar with JavaScript.

Node.js is widely used for building web servers, network applications, command-line tools, and more.