AngularJS is a JavaScript-based open-source front-end web application framework. AngularJS aims to simplify the development of dynamic web applications by providing a structured framework and a set of tools for building single-page applications (SPAs).

AngularJS follows the Model-View-Controller (MVC) architectural pattern, where the application logic is separated from the presentation layer. It uses HTML as the template language and extends its syntax with additional attributes and directives. AngularJS allows developers to build interactive and responsive web applications by declaratively defining the behavior of the user interface.

Key features of AngularJS include:

1. Two-way data binding: AngularJS provides automatic synchronization of data between the model (data) and the view (UI), allowing changes in one to be reflected in the other without explicit manipulation.

2. Dependency injection: AngularJS has a built-in dependency injection mechanism that simplifies the management of dependencies between different components of an application. This promotes modularity and reusability of code.

3. Directives: AngularJS introduces custom HTML directives that extend the functionality of HTML and enable the creation of reusable UI components. Directives allow developers to manipulate the DOM (Document Object Model) and add dynamic behavior to elements.

4. Services: AngularJS offers a range of built-in services that provide common functionality, such as HTTP communication, routing, and data storage. Services can be easily injected into components and used to implement specific business logic.

5. Testing: AngularJS provides robust support for unit testing. It includes features like dependency injection, mocking, and test runners, making it easier to write testable code and automate the testing process.