NodeJS is a server side scripting platform, available as Command Line Interface [command line tool]. It is built with V8 Chrome Engine and LibUV [a c library for Async I/O].
As it uses V8 as runtime, we have to use JS as development language. It is used to create the application which expects Non Blocking I/O, scale-ability, servers with low latency.
We can use NodeJS to create JS CMD programs, web servers, file servers, real time applications like Chat apps, stock monitors.