A Hitchhiker's Guide to NodeJS - part of the Hitchhiker series of tips, guides, tricks and more that we've learned or found useful as we've gone along
In short, it's an awesome runtime server for Javascript applications that has huge developer adoption because it's awesome.
If you want thier official statement, or to learn more, visit the Official NodeJS page
In the meantime, here's some stuff we wrote that you might find helpful when playing with NodeJS