Apache Struts is an open-source, Model-View-Controller (MVC) framework for building Java-based web applications. It provides a set of reusable components, such as controllers, form validation, and view components, that simplify web application development. Struts has been widely used in the development of enterprise-level web applications, and it supports various technologies like HTML, XML, JSP, and Servlet. It is considered a specialized skill because it requires a deep understanding of web application development and Java.