OWL, the "Web Ontology Language", is a language for describing ontologies. "OWL builds on RDF and RDF Schema and adds more vocabulary for describing properties and classes".
Let's say you have a concept of location. You could say in your language: "Santa-Cruz is-a City. Santa-Cruz is-located-in California. California is-located-in United-States-of-America. United-States-of-America is-located-in Earth."
Now, can a computer know that "Santa-Cruz is-located-in Earth?" Theoretically, yes, but you have to give it a piece of information: "if A is-located-in B, and if B is-located-in C, then A is-located-in C." (Math people call this the "transitive property.")
Once you've told a computer that A-in-B & B-in-C means A-in-C, then it can figure out that Santa-Cruz is-located-in the-World.
OWL is the language in the W3C's Semantic Web suite that takes care of saying things like "A-in-B & B-in-C means A-in-C."
See also: ESW:FormalSystemsLanguage
Ah, what the hell- I'll just link it right here:
Makes more sense when you have the accompanying text though- read it at ESW:FormalSystemsLanguage