AngularJS is a client-side JavaScript MVC framework that extends HTML with new attributes and custom tags. It allows developers to create reusable components with modularity, dependency injection, two-way data binding, and testing. Key concepts include expressions, filters, directives, models, modules, controllers, services, and templates. Modules allow building reusable components like controllers, services, filters and directives. Controllers set up initial scope state and behavior while services extract reusable code and enable communication between controllers. Two-way data binding synchronizes models and views.