AngularJS is an MV* framework that is ideal for use when building client-side single-page apps. It is not a library, but a framework for building dynamic web pages. It focuses on extending HTML and providing dynamic data binding, and it plays well with other frameworks (e.g., jQuery).
If you are building a single-page app, AngularJS will be perfect for you. Gmail, Google Docs, Twitter, and Facebookall fit into the AngularJS sweet spot. Game development and other apps that heavily manipulate the DOM or need pure speed are not good fits for AngularJS.