Node.js is primarily a JavaScript runtime environment but it's much more than just that; it's one of the reasons for the immense popularity of JavaScript. Node takes JavaScript out of the browser and puts it in the environment of an operating system where it can read files, create servers, take input from the terminal, and much more.
I've created this mini course to get you quickly up and running with Node. The focus throughout the course is to expose you to the most important parts of Node and omit the unnecessary details so that you can pick up the concepts in almost no time. To learn more advanced ideas, you're encouraged to refer to Advanced Node.js.