AngularJS is an open-source JavaScript framework for building single-page applications. The document introduces AngularJS and its core concepts including directives, data binding, modules, controllers, dependency injection and services. It provides code examples for common directives, controllers, services and best practices for file organization. The latter part of the document outlines a tutorial for building an AngularJS app step-by-step including setting up the app module, adding controllers, using ng-repeat and filters, and implementing a service.