From the struts-documentation: Tiles is a templating framework designed to easily allow the creation of web application pages with a consistent look and feel. It can be used for both page decorating and componentization.
Or, from the tiles homepage (http://tiles.apache.org/): Apache Tiles is a templating framework built to simplify the development of web application user interfaces. Tiles allows authors to define page fragments which can be assembled into a complete page at runtime. These fragments, or tiles, can be used as simple includes in order to reduce the duplication of common page elements or embedded within other tiles to develop a series of reusable templates. These templates streamline the development of a consistent look and feel across an entire application. Tiles grew in popularity as a component of the popular Struts framework. It has since been extracted from Struts and is now integrated with various frameworks, such as Struts 2 and Shale.