What is Node.js and what makes it different from other server-side technologies?

Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine that allows JavaScript to run on the server-side. Key differences: 1) Single-threaded with event loop, 2) Non-blocking I/O operations, 3) Uses JavaScript for both client and server, 4) NPM package ecosystem, 5) High performance for I/O intensive applications.