Node.js is a web application framework built on Google Chrome's JavaScript Engine(V8 Engine).
Node.js comes with RUNTIME environment on which a Javascript based script can be interpreted and executed (It is analogus to JVM to JAVA BYTE code). This runtime allows to execute a JavaScript code on any machine outside a browser. Because of this runtime of Node.js, JavaScript is now can be executed on server as well.
Node.js also provides a rich library of VARIOUS javascript modules which eases the developement of web application USING Node.js to great extents.
Node.js = Runtime Environment + JavaScript Library

Node.js is a web application framework built on Google Chrome's JavaScript Engine(V8 Engine).
Node.js comes with runtime environment on which a Javascript based script can be interpreted and executed (It is analogus to JVM to JAVA byte code). This runtime allows to execute a JavaScript code on any machine outside a browser. Because of this runtime of Node.js, JavaScript is now can be executed on server as well.
Node.js also provides a rich library of various javascript modules which eases the developement of web application using Node.js to great extents.
Node.js = Runtime Environment + JavaScript Library