AngularJS is an open-source framework developed by Google in 2009 for creating dynamic web apps using HTML as its template language, but it is no longer being developed as Angular (from Angular 2 onward) has replaced it with significant improvements including the use of TypeScript. The key differences between AngularJS and Angular include architecture, language compatibility, mobile support, and dependency injection systems. While AngularJS is still useful for certain applications, Angular provides a more structured and maintainable framework for modern web development.