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1. Introduction

"Tell me what wines I should buy to serve with each course of the following menu. And, by the way, I don't like Sauternes."
It would be difficult today to construct a Web agent that would be capable of performing a search for wines on the Web that satisfied this query. Similarly, consider actually assigning a software agent the task of making a coherent set of travel arrangements. (For more use cases see the OWL requirements document (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#Requirements).)
To support this sort of computation, it is necessary to go beyond keywords and specify the meaning of the resources described on the Web. This additional layer of interpretation captures the semantics of the data.
The OWL Web Ontologoy Language is a language for defining and instantiating Web ontologies. Ontology is a term borrowed from philosophy that refers to the science of describing the kinds of entities in the world and how they are related. An OWL ontology may include descriptions of classes, properties and their instances. Given such an ontology, the OWL formal semantics (http://www.w3.org/TR/2004/REC-owl-semantics-20040210/) specifies how to derive its logical consequences, i.e. facts not literally present in the ontology, but entailed by the semantics. These entailments may be based on a single document or multiple distributed documents that have been combined using defined OWL mechanisms (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#import).
This document is one component of the description of OWL, the Web Ontology Language, being produced by the W3C Web Ontology Working Group (WebOnt). The Document Roadmap (http://www.w3.org/TR/2004/REC-owl-features-20040210/#s1.1) section of the Overview ([Overview (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#FeatureList)], 1.1) describes each of the different parts and how they fit together.
One question that comes up when describing yet another XML/Web standard is "What does this buy me that XML and XML Schema don't?" There are two answers to this question.

• An ontology differs from an XML schema in that it is a knowledge representation, not a message format. Most industry based Web standards consist of a combination of message formats and protocol specifications. These formats have been given an operational semantics, such as, "Upon receipt of this PurchaseOrder message, transfer Amount dollars from AccountFrom to AccountTo and ship Product." But the specification is not designed to support reasoning outside the transaction context. For example, we won't in general have a mechanism to conclude that because the Product is a type of Chardonnay it must also be a white wine.
  
  

• One advantage of OWL ontologies will be the availability of tools that can reason about them. Tools will provide generic support that is not specific to the particular subject domain, which would be the case if one were to build a system to reason about a specific industry-standard XML schema. Building a sound and useful reasoning system is not a simple effort. Constructing an ontology is much more tractable. It is our expectation that many groups will embark on ontology construction. They will benefit from third party tools based on the formal properties of the OWL language, tools that will deliver an assortment of capabilities that most organizations would be hard pressed to duplicate.
  
  

1.1. The Species of OWL

The OWL language provides three increasingly expressive sublanguages designed for use by specific communities of implementers and users.

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OWL Lite supports those users primarily needing a classification hierarchy and simple constraint features. For example, while OWL Lite supports cardinality constraints, it only permits cardinality values of 0 or 1. It should be simpler to provide tool support for OWL Lite than its more expressive relatives, and provide a quick migration path for thesauri and other taxonomies.

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OWL DL supports those users who want the maximum expressiveness without losing computational completeness (all entailments are guaranteed to be computed) and decidability (all computations will finish in finite time) of reasoning systems. OWL DL includes all OWL language constructs with restrictions such as type separation (a class can not also be an individual or property, a property can not also be an individual or class). OWL DL is so named due to its correspondence with description logics [Description Logics (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#DescriptionLogics)], a field of research that has studied a particular decidable fragment of first order logic. OWL DL was designed to support the existing Description Logic business segment and has desirable computational properties for reasoning systems.

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OWL Full is meant for users who want maximum expressiveness and the syntactic freedom of RDF with no computational guarantees. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right. Another significant difference from OWL DL is that a owl:DatatypeProperty can be marked as an owl:InverseFunctionalProperty. OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. It is unlikely that any reasoning software will be able to support every feature of OWL Full.
Each of these sublanguages is an extension of its simpler predecessor, both in what can be legally expressed and in what can be validly concluded. The following set of relations hold. Their inverses do not.

• Every legal OWL Lite ontology is a legal OWL DL ontology.
  
• Every legal OWL DL ontology is a legal OWL Full ontology.
  
• Every valid OWL Lite conclusion is a valid OWL DL conclusion.
  
• Every valid OWL DL conclusion is a valid OWL Full conclusion.
  

Ontology developers adopting OWL should consider which species best suits their needs. The choice between OWL Lite and OWL DL depends on the extent to which users require the more expressive restriction constructs provided by OWL DL. Reasoners for OWL Lite will have desirable computational properties. Reasoners for OWL DL, while dealing with a decidable sublanguage, will be subject to higher worst-case complexity. The choice between OWL DL and OWL Full mainly depends on the extent to which users require the meta-modelling facilities of RDF Schema (i.e. defining classes of classes). When using OWL Full as compared to OWL DL, reasoning support is less predictable. For more information about this issue see the OWL semantics document (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#FormalModel).
Users migrating from RDF to OWL DL or OWL Lite need to take care to ensure that the original RDF document complies with the constraints imposed by OWL DL and OWL Lite. The details of these constraints are explained in Appendix E (http://www.w3.org/TR/2004/REC-owl-ref-20040210/#app-DLinRDF) of the OWL Reference.
When we introduce constructs that are only permitted in OWL DL or OWL Full, they are marked by "[OWL DL]".

1.2. Structure of the Document

In order to provide a consistent set of examples throughout the guide, we have created a wine (http://www.w3.org/TR/2004/REC-owl-guide-20040210/wine.rdf) and food (http://www.w3.org/TR/2004/REC-owl-guide-20040210/food.rdf) ontology. This is an OWL DL ontology. Some of our discussion will focus on OWL Full capabilities and is so marked. The wine and food ontology is a significant modification of an element (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#wine1) of the DAML ontology library with a long history. It was originally developed by McGuinness as a CLASSIC description logic example (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#wine3), expanded to a description logic tutorial (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#wine4), and expanded to an ontology tutorial (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#wine2).
In this document we present examples using the RDF/XML syntax (http://www.w3.org/TR/REC-rdf-syntax/#grammar) ([RDF (http://www.w3.org/TR/2004/REC-owl-guide-20040210/#RDF1)], 5), assuming XML will be familiar to the largest audience. The normative OWL exchange syntax is RDF/XML. Note that OWL has been designed for maximal compatibility with RDF and RDF Schema. These XML and RDF formats are part of the OWL standard.
All of the examples presented in this document are taken from the ontologies contained in wine.rdf (http://www.w3.org/TR/2004/REC-owl-guide-20040210/wine.rdf) and food.rdf (http://www.w3.org/TR/2004/REC-owl-guide-20040210/food.rdf), except those marked with  ?  in the bottom right corner.

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