Is it a web server or a programming language for server-side scripts?
From Node.js website
Event-driven means that the server only reacts when an event occurs. This allow us to create high performance, highly scalable, “real-time” applications.
Finally, this is probably the best article that will get you excited about Node.js
Directly from the node.js tag wiki, make sure watch some of the talk videos linked there to get a better idea.