AngularJS is an open-source, front-end web application framework developed by Google. It allows developers to build dynamic, single-page web applications (SPAs) using HTML, CSS, and JavaScript. AngularJS extends HTML with directives and two-way data binding, which automatically updates the user interface when the model data changes. It simplifies tasks like handling forms, routing, and managing dependencies, and supports modular development through services and controllers. AngularJS helped popularize the MVC (Model-View-Controller) architecture for web development and is known for making complex web applications more maintainable and scalable.