Jolie O'Dell writes about Node on Mashable
In traditional languages and frameworks, the communication inside the app between the web server and the database is the most time-intensive part of the transaction. Node makes a much smaller footprint on your web server. It allocates web server resources on an as-needed basis, not pre-allocating a large chunk of resources for each user. For example, Apache might assign 8MB to a user, while Node assigns 8KB. “The way that Node is more efficient on servers is by not allocating resources to things while it waits,” says Hughes-Croucher.
“Say you have to talk to the database, and that’s going to take 50ms to respond. Instead of assigning all of the processing resources for that 50ms wait, it just uses a placeholder. When the database responds, then it allocates the resources needed to process. That means it’s totally possible to do a lot more requests at once, because you only allocate the server resources when you need to use them, not while you are waiting on databases.”
as well as an article by Noah Gift and Jeremy Jones: Use Node.js as a full cloud environment development stack