Internet marketplaces are now faced with new challenges that arise from the need to seamlessly integrate enormous number of product catalogs from different sources. In order to help users find products efficiently, Internet shops provide hierarchies of product catalogs (called category hierarchies). However, the absence of robust models (and well understood semantics) for product catalogs and their hierarchies severely impairs our ability to systematically support structured operations. In this paper, we present an extended catalog model whose purpose is to provide a universal product catalog repository with browsing, searching, and downloading capabilities in order to facilitate catalog sharing and interoperability. In addition, we present a model for category hierarchies with well defined semantics which allows flexible representation of product hierarchies as well as automatic merging of different category hierarchies as a special case of catalog integration.