There’s a nice analogy of this described here:
Relevant text from the above link:
Explain “Event-Driven” Web Servers to Your Grandma You’ve heard the
term event-driven, event-based, or evented when it comes to web
servers. Node.js is based on evented I/O. nginx is an asynchronous
event-driven web server.But what does the term mean? Here’s an easy way to think about it.
Let’s think of a web server as a pizza shop that needs to take orders
over the phone (requests for web pages).Traditional Web Server
The pizza shop hires operators (processes/threads) to take orders over
the phone. Each operator has one phone line. After the operator is
done taking the order, the operator keeps the customer on the line
until the pizza (response web page) is done baking and then tells them
it’s ready to pick up.So the pizza shop needs to hire as many operators as the number of
pizzas that may be baked at once in order to serve all customers who
call in.Event-Driven Web Server
The pizza shop only hires one operator, but has trained the operator
to hang up after taking the order, and call the customer back when the
pizza is ready to be picked up.Now one operator can serve many customers.