Node.js is a server-side JavaScript environment that uses an asynchronous event-driven model. Node is built on top of Google’s V8 JavaScript engine. Google V8 actually compiles JavaScript into native machine code prior to execution, resulting in extremely fast runtime performance. Node enables you to rapidly build network apps that are lightning fast and highly concurrent.

Node.js and “event-driven” programming is best explained by this post using two analogies

Related Links:

1. Beginners Guide
2. Bad and Good Use cases for Node.js
3. Understanding node.js
4. Node.js for Java developers