An ontology language is a special case of a formal language which is commonly used to specify ontologies. Note that this category should contain contrete instances of ontology lanugages, but not everything that is generally related to such languages.
 What is an ontology language?
The term "ontology" itself is rather broad, so for clarification, every page about an ontology language should provide a link to at least one ontology document generated with that language. Note that ontology languages are distinct from markup languages (though some rare languages might be argued to be both), query languages, and other types of formal languages.
Moreover, there are a number of vocabularies that are also used in many ontologies, but which are not "languages" in a strict sense. This category is intended for basic languages like RDF and OWL, which typically comprise of a formally specified syntax and semantics, not for all vocabularies based on these languages. Yet, it might sometimes be in the eye of the beholder whether some formalism is an independent ontology language or not, and in general practical usage may be most important.
Things like RDFa and Microformats can be classified here, even though they are very light-weight indeed. Sub-classifying into "schema langauges" and "data languages" would be possible to elaborate the differences further.