Geographical and geospatial ontologies are receiving a considerable attention in information technology, due to three different factors: the growing diffusion of Geographic Information Systems (GIS), their use in different applications and the impulse given by Semantic Web to this research area. The aim of these pages is to describe what a geo-information ontology is, in order to systematize the contemporary debate. In the first part, we delineate the domain of ontology of geography within the contemporary philosophical context. In the second part, we define the meaning of ontology in the information technology domain. In the last two parts, finally, we analyze contemporary geo-information ontologies, suggesting a potential classification.