AngularJS

AngularJS is an open-source JavaScript framework, maintained by Google. Its goal is to augment browser-based applications with model–view–controller (MVC) capability, in an effort to make both development and testing easier.

Why AngularJS:

HTML is great for declaring static documents, but it falters when we try to use it for declaring dynamic views in web-applications. AngularJS lets to extend HTML vocabulary for the application. The resulting environment is extraordinarily expressive, readable, and quick to develop. AngularJS is a toolset for building the framework most suited to application development. It is fully extensible and works well with other libraries.

Benefits:

• Uses primitive JavaScript types instead of function wrappers
• Don't need to write all the event listening and event triggering, loaded with functionality
• Decouple the client side of an application from the server side. This allows development work to progress in parallel, and allows for reuse of both sides.
• Decouple DOM manipulation from application logic. This improves the testability of the code.
• Fast development and makes testing easy and offers many testing tools
• Out of the box features: Data binding, Data filtering/sorting, Data templates, Ajax support, Form validation, Routing, History support, Deep linking, Modular and reusable components, Loosely coupled architecture (dependency injection), MVC-style applications, Architected with unit testing in mind
• AngularJS liberates from having to do the following:
• Register callbacks, Manipulate the DOM programmatically
• Move data in and out of the UI manually with code
• Writing a bunch of initialization code just to get started

Liabilities:

• Need to invest time to understand the DOM and angular syntax properly.
• Writing directives is not easy. It’s the most complex part of writing Angular code.
• Form validation framework of angular is not perfect yet. It still needs some tweaks.
• Clutters the global namespace with every single controller that we add and JavaScript errors happen if we don’t clutter the window object.

Comparison with Backbone.js: Backbone.js is a very lightweight, which means we have to type more, but might be a good fit for a broader spectrum of apps. Backbone allows to structure the JavaScript code in an MVC fashion. AngularJS is way bigger, provides a much higher level of abstraction and is very opinionated on how we should develop an app. It's a complete solution, with testing environment and philosophy behind it that means, building an app is super-fast, but it doesn't have to fit everyone nor every app.
More on http://angularjs.org/