On the Semantic Web, vocabularies define the concepts and relationships (also referred to as “terms”) used to describe and represent an area of concern. Vocabularies are used to classify the terms that can be used in a particular application, characterize possible relationships, and define possible constraints on using those terms. In practice, vocabularies can be very complex (with several thousands of terms) or very simple (describing one or two concepts only).
There is no clear division between what is referred to as “vocabularies” and “ontologies”. The trend is to use the word “ontology” for more complex, and possibly quite formal collection of terms, whereas “vocabulary” is used when such strict formalism is not necessarily used or only in a very loose sense. Vocabularies are the basic building blocks for inference techniques on the Semantic Web.
Source: https://www.w3.org/standards/semanticweb/ontology
The most commonly used Ontologies are schemaorg which is most notably used for powering search, alongside Open Graph Protocol, which is used to support the means through which web-content can be reposted on facebook.
Beyond these two notable examples, an array of public ontologies can be found through sites such as The Linked Open Data Cloud site.