Node is a platform built to run JavaScript on the server. What that means is, the same JavaScript that powers dynamic frontend applications can also now be used to handle server-side code.

Officially, Node.js is defined thus:

Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine

Important to note here is Node.js is a runtime and it comprises of the V8 JavaScript engine which is also used by Chrome browser, and in addition, it has some libraries built into it that makes it ready as a server-side platform.

Node is written in C++ programming language.

Node.js is useful in a number of real-world applications. For building JSON APIs, Single page Applications, Data Streaming Applications, I/O applications (it can also interact with the file systems and Operating system) etc.

Node.js is Asynchronous, Event driven, fast and is single-threaded with event-looping.