There is a very good fast food place analogy that best explains the event driven model of Node.js, see the full article, Node.js, Doctor’s Offices and Fast Food Restaurants – Understanding Event-driven Programming
Here is a summary:
If the fast food joint followed a traditional thread-based model, you'd order your food and wait in line until you received it. The person behind you wouldn't be able to order until your order was done. In an event-driven model, you order your food and then get out of line to wait. Everyone else is then free to order.
Node.js is event-driven, but most web servers are thread-based.York explains how Node.js works:
You use your web browser to make a request for "/about.html" on a
Node.js web server.
The Node.js server accepts your request and calls a function to retrieve
that file from disk.
While the Node.js server is waiting for the file to be retrieved, it
services the next web request.
When the file is retrieved, there is a callback function that is
inserted in the Node.js servers queue.
The Node.js server executes that function which in this case would
render the "/about.html" page and send it back to your web browser."
