What is Node.js?

Node.js is a 

• open-source 
• cross-platform 
• JavaScript run-time environment 
• build on google chrome's JavaScript engine(V8) 

for developing server site application. 

Node.js = Runtime Environment + JavaScript Library

Node.js uses
an asynchronous
event driven 
non-blocking I/O
model that makes it light weight, efficient and perfect for data-intensive real time application that runs on distributed devices.