Background and intro

An ontology is about:

explicit meanings and relationships; the terms used are less important
a combination of definitions in both text and logic

An ontology can be the basis of, but is broader than:

a taxonomy
a vocabulary
a data or object model
a conceptual model
a specific serialization format

Ontologies can be expressed at different levels of sophistication, with different scopes, and in a combination of languages. The basic structures include:

individual: a representation of a business object or item which is the subject of information to be managed. An individual has a unique identity. For example Person X or Shipment Y. Many such individuals might represent the same real world object.
data value: strings, numbers, dates which represent the data.
property: a type of data point that may be associated with individuals. An individual, a property and a value --- which may be a data value or another individual --- form a triple.
For example person X hasBirthDate D or person X hasMother Y. Triples whose value is another individual form relationships. Properties may have generalizations, for example hasMother is a subProperty of hasParent.
class: a category applied to individuals, that determines what you can do with them, the properties you can expect to see, and the rules that might apply; an individual may be a member of many classes associated; classes may have generalizations. Note that, unlike more traditional approaches, properties are independent of classes or class membership. For example, given the triple X hasMother Y you may be able to infer that both X and Y are members of the class Person, or at least Animal.
ontology: grouping of the above for management and identification purposes.