Node.js can be used for a wide variety of purposes. Because it is based on V8 and has highly optimized code to handle HTTP traffic, the most common use is as a webserver. However, Node.js can also be used for a variety of other web services, such as:
Web services APIs such as REST
Real-time multiplayer games
Backend web services such as cross-domain, server-side requests
Web-based applications
Multiple-client communication such as IM