[Node.js][1] is an event based, asynchronous I/O framework that runs on top of Google's [V8 JavaScript engine][2].
Node.js - or just Node as it's commonly called - is used for developing applications that make heavy use of the ability to run [JavaScript][3] both on the client, as well as on server side and therefore benefit from the re-usability of code and the lack of context switching.
It's also possible to use majored JavaScript frameworks like [YUI][4] and [jQuery][5] for server side DOM manipulation.
To ease the development of complex JavaScript further, Node.js supports the [CommonJS][6] standard that allows for modularized development and the distribution of software in packages via the [Node Package Manager][7].
Applications that can be written using Node.js include (but are not limited to):