In this paper we develop a framework for comparing ontologies, and place a number of the more prominent ontologies into it. We have selected 10 specific projects for this study, including general ontologies, domain specific ones, and one knowledge representation system. The framework includes general characteristics such as the purpose of an ontology, its coverage (general or domain-specific), its size, and the formalism used. It also includes the design process used in creating an ontology and the methods used to evaluate it. Characteristics that describe the content of an ontology include taxonomic organization, types of concepts covered, top-level divisions, internal structure of concepts, representation of part-whole relations, and the presence and nature of additional axioms. Finally we consider what experiments or applications have used the ontologies. Knowledge sharing and reuse will require a common framework to support interoperability of independently created ontologies. Our study shows there is great diversity in the way ontologies are designed and the way they represent the world. By identifying the similarities and differences among existing ontologies, we clarify the range of alternatives in creating a standard framework for ontology design.