basics of Node.js

I am a newbie in this kind of stuff but lately I’ve been hearing a lot about how good NodeJS is. Considering how much I love working with jQuery and Javascript in general, I can’t help but wonder how to decide when to use NodeJS. The web application I have in mind is something like bit.ly – takes some content, archives it.
From all the homework I have been doing in the last few days, I obtained the following information. NodeJS
  • is a command-line tool that can be run as a regular web server and lets one run Javascript programs
  • utilizes the great V8 JS engine
  • is very good when you need to do several things at the same time
  • is event-based so all the wonderful Ajax like stuff can be done on the server side
  • lets us share code between the browser and the backend
  • lets us talk with MySQL

Some of the sources that I have come across are:

Considering that NodeJS can be run almost out-of-the-box on Amazon’s EC2 instances, I am trying to understand what type of problems require NodeJS as opposed to any of the mighty kings out there like php, python and ruby. I understand that it really depends on the expertise one has on a language but my question falls more into the general category of: When to use a particular framework and what type of problems is it particularly suited for?