What is Node.js?

Node.js is a server-side javascript interpreter that is useful for receiving and responding to http requests. Node itself isn't javascript, but the code that it handles is javascript. Node is actually a c program, and uses javascript as a way to send instructions to the c program and drastically simplify the server networking code required in c.

source: http://radar.oreilly.com/2011/07/what-is-node.html