Node.js is a JavaScript runtime environment for building fast and scalable network applications. It uses non-blocking I/O and event-driven architecture, allowing it to handle multiple connections simultaneously without blocking. This makes Node.js suitable for real-time web applications with many simultaneous connections. Node.js has a small core but is extensible through modules. Common modules include Express for building web servers and Socket.IO for real-time web functionality. While Node.js is well-suited for I/O-intensive applications, it is less suitable for CPU-intensive or memory-intensive applications due to its single-threaded nature.