Node.js learning resources
How node.js event loop works
If you want to dig little deeper into node.js (I highly recommend you to do that) and learn how it works there are several articles on this topic like Understanding the node.js event loop and Understanding event loops and writing great code for Node.js.
Event driven programming, callbacks and asynchronous code design
Event driven programming (and architecture) represents a core concept for writing node.js programs and you should read the following articles not only to understand event driven systems, but also to learn how and why you should use callbacks and take advantage of asynchronous code: Node.js, Doctor’s Offices and Fast Food Restaurants – Understanding Event-driven Programming and Asynchronous Code Design with Node.js.