JavaServer Faces (JSF) is a Java-based web application framework intended to simplify the development of user interfaces (UI) for Java EE applications.

Key concepts and features of JSF include:

1. Component-Based UI: JSF uses a component-based approach to building UI, much like Swing does for desktop applications. This means the interface can be created using pre-built and custom-built UI components, such as text boxes, buttons, tables, etc.

2. Event Driven Programming Model: JSF has a built-in event handling mechanism for managing events such as button clicks or data changes, etc.

3. Navigation Model: JSF has a navigation model which can be used to define the sequence in which pages are displayed in a multi-page application.

4. Validation, Conversion, and Error Handling: JSF provides a robust mechanism to perform data validation, conversion, and error handling.

5. Integration with other Java EE Technologies: JSF can be easily integrated with other Java technologies like Servlets, JSP, EJB, JPA, etc.

6. Dependency Injection: JSF 2.0, released in 2009, introduced built-in support for Dependency Injection (DI) to allow for more flexible and testable code.

7. Ajax Support: JSF 2.0 also introduced built-in Ajax support. This enables you to create interactive web applications without needing to write a lot of JavaScript.

8. Templating: JSF includes Facelets, a powerful templating system, which allows for re-use of layouts and designs.

Since JSF is a specification, there are several implementations available, such as Mojarra (the reference implementation provided by Oracle) and MyFaces (an Apache Foundation project).