Sometimes we use to confuse formal ontologies with terminological systems. All systems are used to organize knowledge by describing universal terms. An ontology is not a vocabulary but it uses a vocabulary of universal terms and relationship terms to formally describe how we imagine different entities in a domain of interest are related or should be related. A formal ontology defines high level concepts (i.e. very generic) and the relationships between them. It consolidates then a structure of interrelated classes that can be used by computer or information scientists with discipline and consistency as common reference in order to map and transform data between schemata. A formal ontology, having defined classes and the properties between them, it can then describe facts (factual data), meaning cases of things and of relationships that exist or have existed in the real world (particulars).
On the other hand we define vocabularies, terminologies, typologies etc as systems of named universals (classes) which are used in a documentation system as data. Terminological systems are organized in KOS. They describe aspects of things, they serve to classify terms typically with specialization/generalization hierarchical relationships and categorical relations. Such knowledge organization systems are very important for assigning subject-oriented data in documentation tasks.