While reading the W3C Recommendation OWL Use Cases and Requirements, I was surprised at how many nice, succinct explanations of basic OWL and ontology-related concepts it had, so I thought I'd reproduce some highlights here. For example, take its definition of an ontology:
An ontology formally defines a common set of terms that are used to describe and represent a domain...An ontology defines the terms used to describe and represent an area of knowledge.
What the hell is a conceptualization?
The most popular definition of ontology is "a formalization of a conceptualization", as if the readers are going to sit back and say "OK! Formalization! Conceptualization! Got it! All set here!", except that they're really thinking "What the hell is a conceptualization?" I much prefer the OWL Use Cases explanation, which while not being as "formal", at least explains what's going on.
Here are some more nice quotes from the Use Cases document, arranged as a fake FAQ:
Why would anyone use an ontology?
Ontologies are used by people, databases, and applications that need to share domain information (a domain is just a specific subject area or area of knowledge, like medicine, tool manufacturing, real estate, automobile repair, financial management, etc.). Ontologies include computer-usable definitions of basic concepts in the domain and the relationships among them...Ontologies can prove very useful for a community as a way of structuring and defining the meaning of the metadata terms that are currently being collected and standardized.
I keep seeing the word "ontology" used to describe different things. What's up with that?
(note that here and throughout this document, [the] definition [of ontology] is not used in the technical sense understood by logicians)... The word ontology has been used to describe artifacts with different degrees of structure. These range from simple taxonomies (such as the Yahoo hierarchy), to metadata schemes (such as the Dublin Core), to logical theories.
What is the Semantic Web, and where do ontologies fit in?
The Semantic Web is a vision for the future of the Web in which information is given explicit meaning, making it easier for machines to automatically process and integrate information available on the Web. The Semantic Web will build on XML's ability to define customized tagging schemes and RDF's flexible approach to representing data. The next element required for the Semantic Web is a web ontology language which can formally describe the semantics of classes and properties used in web documents. In order for machines to perform useful reasoning tasks on these documents, the language must go beyond the basic semantics of RDF Schema.
How can OWL do this?
The Web Ontology Working Group charter tasks the group to produce this more expressive semantics and to specify mechanisms by which the language can provide "more complex relationships between entities including: means to limit the properties of classes with respect to number and type, means to infer that items with various properties are members of a particular class, a well-defined model of property inheritance, and similar semantic extensions to the base languages."
The Semantic Web needs ontologies with a significant degree of structure. These need to specify descriptions for the following kinds of concepts:
Classes (general things) in the many domains of interest
The relationships that can exist among things
The properties (or attributes) those things may have
I don't know much about RDF Schema. What does it let you do, and what does OWL add?
With RDF Schema, one can define classes that may have multiple subclasses and super classes, and can define properties, which may have sub properties, domains, and ranges. In this sense, RDF Schema is a simple ontology language. However, in order to achieve interoperation between numerous, autonomously developed and managed schemas, richer semantics are needed. For example, RDF Schema cannot specify that the Person and Car classes are disjoint, or that a string quartet has exactly four musicians as members.
The whole Uses Cases document is worth checking out.