What is meant by Ontology?

An ontology refers to a model that contains a set of classes (individuals) plus a set of relations between them.

Lets consider a relational database model, having three tables: Human, Man, Woman. As you can observe, there is no relation defined between them.
Well, in an ontology, besides that, you might also have a set of rules like:
- Object X is Man, if and only if is Human, but is not Woman.
- If object X is Human, then it is either Man or Woman, but not both.

For defining ontologies there are several languages developed, the most representative being OWL (Web Ontology Language. And for querying them one must also use a specialized language (eg. SPARQL).