Node.js is a framework for dynamic websites

Node.js is a framework that is based on what is currently the fastest JavaScript compiler, Google V8. Node.js focuses on the integration of external resources, such as a network or file system. Node.js can be used for dynamic websites with high database queries, large websites with lots of traffic and browser games and web applications. Because Node.js is based on JavaScript, it is relatively easy for web developers to begin working with it. Node.js is an interesting technology which allows you to develop complex and highly scalable web applications with a few lines of code. Node.js also lets you develop server-side web applications. Node.js contains the http-module for this reason, which provides features for developing web servers. Node.js is platform-independent, and runs on OS X and Linux, as well as Windows. The developer is free to choose a suitable development platform.

Node.js is based on JavaScript and easy to learn

A great strength of Node.js is found in the easy programming of scalable applications in a language, which is familiar to any web developer. Further advantages and strengths of Node.js lie in its quick and high-performance execution, in its relatively low memory utilisation and in the possibility to use common code on server and client (i.e. JavaScript).

You can find more information on Node.js on Wikipedia:

And on the official website of Node.js: