Node JS is an open-source, cross-platform, back-end JavaScript runtime environment that runs on the Chrome V8 JavaScript engine and executes JavaScript code outside a web browser.
We use Node JS to build backend services for web applications (also referred to as Application Programming Interfaces or APIs). Node JS is ideal for building highly scalable, data-intensive, real-time backend services that power client applications.
The use of the Chrome V8 Javascript engine enables Node JS to be a high performer.
Some advantages Node JS has include ease of use making it ideal for prototyping and for agile development. It's a open-source programming language and it has a big community, it has a vast array of open source libraries and by primarily using Javascript it ensures that a full-stack developer is able to write cleaner and more consistent code across their application. 
Some major corporations that use Node JS in their stack include Netflix, Paypal, NASA, eBay among numerous others. 
Below is the code to a simple Hello World program that runs on port 3000 on your localhost server.

A Node JS app runs in a single process, without creating a new thread for every request. Node JS provides a set of asynchronous I/O primitives in its standard library that prevent JavaScript code from blocking and generally libraries in Node.js are written using non-blocking paradigms, making blocking behavior the exception rather than the norm.
When Node.js performs an I/O operation, like reading from the network, accessing a database or the filesystem, instead of blocking the thread and wasting CPU cycles waiting, Node.js will resume the operations when the response comes back.
This allows Node.js to handle thousands of concurrent connections with a single server without introducing the burden of managing thread concurrency which could be a significant source of bugs.