Node.js, created by Ryan Dahl and first released in 2009, is an open-source server environment designed to run on multiple platforms, including Windows, Linux, Unix, and macOS. It is a back-end JavaScript runtime environment that uses the V8 JavaScript engine to execute JavaScript code outside of a web browser. With Node.js, developers can create command-line tools and server-side scripts using JavaScript. By enabling scripts to run server-side, Node.js allows for the creation of dynamic web page content before it is sent to the user's browser. This approach unifies web application development by promoting the use of a single programming language across both server-side and client-side scripts.