Node.js is an open source JavaScript runtime environment for developing server-side applications. It allows JavaScript to be used for non-browser programs like web servers. Node.js uses asynchronous, event-driven programming and non-blocking I/O calls which makes it lightweight and efficient for data-intensive real-time applications. It runs the V8 JavaScript engine and can generate dynamic page content, work with files and databases, and perform tasks like adding/modifying data. The document then discusses features of Node.js like asynchronous programming, speed, scalability, and licensing, as well as its advantages and disadvantages compared to traditional web servers.