AngularJS is an open-source JavaScript framework developed by Google for building single-page web applications, notable for its ability to enhance HTML with directives and data binding without needing page reloads. It features a modular architecture, allowing for the organization of controllers, services, and directives, with initialization directives like ng-app and ng-init to set up the application. Controllers manage application behavior using the $scope object, and services provide reusable functions for various tasks throughout the app.