It's built to handle asynchronous I/O from the ground up and is a good match to a lot of common web- and network-development problems.  In addition to fast JavaScript execution, the real magic behind Node.js is called the Event Loop. To scale to large volumes of clients, all I/O intensive operations in Node.js are performed asynchronously.
Node.js is Javascript.  So the same language can be used on the backend and frontend.   This means it breaks down the boundaries between front- and back-end development. In addition to it’s innate capabilities, Node.js has a thriving open source community which has produced many excellent modules to add additional capabilities to Node.js applications. One of the most famous is Socket.io, a module to manage persistent connections between client and server, enabling the server to push real-time updates to clients. Socket.io abstracts the technology used to maintain these connections away from the developer, automatically using the best technology available for a particular client (websockets if the browser supports it, JSONP or Ajax longpolling if not).