Struts is an open-source web application framework for building Java-based web applications. It is based on the Model-View-Controller (MVC) architectural pattern and is used to create enterprise-level web applications.
Struts is built on top of the Java Servlet API and use a combination of JavaBeans, JSP, and XML to define the application's structure and behavior. It provides a set of pre-built components, such as ActionForm, Action, and ActionMapping, that can be used to quickly create web applications. It also provides a framework for handling user input, validation, and error handling, as well as integration with other Java technologies such as JavaServer Pages (JSP), JavaServer Faces (JSF), and Java Persistence API (JPA)
Struts also provide a plugin architecture that allows developers to easily extend the framework's functionality. This allows developers to add new features, such as custom tags, to the framework.
Struts has been widely used for building enterprise-level web applications for many years. However, its usage has decreased in recent years as other frameworks such as Spring, Spring MVC, and Play! The framework has become more popular. Nevertheless, Struts is still being used and maintained by a community of developers, and is still a good choice for projects that need a mature and solid framework.